11 ms·

Why 'gallons per mile' is better than 'miles per gallon'

115 points by squeakynick 13y ago

deleted 13y ago
[deleted]
anigbrowl 13y ago
I'll settle for reciprocal distortion if more people would switch to the metric system.
- dredmorbius 13y ago
  Most metric countries report liters/100km, solving both problems at the same time.
  - ProblemFactory 13y ago
    An useless but fun observation: you can do the litres/100km division and arrive at units of area. A family car might have a fuel consumption of 0.05 mm^2. This is the cross-section of the imaginary pipe of fuel along the highway that a car with no fuel tank would have to pick up as it travels.
    - keule 13y ago
      https://what-if.xkcd.com/11/ https://what-if.xkcd.com/11/
    - dredmorbius 13y ago
      Actually, what you're arriving at is energy use per unit distance. The unit of area result you're getting (correcting from original content) is a misuse of units: "liters/100km" is more properly "liters of fuel/100km", where liters-of-fuel is a shorthand for quantity (that is, mass) of fuel used per 100km -- it's more convenient to measure fluid volume than mass (though you'll find that aircraft will measure fuel by weight). And that is still a shorthand for "energy contained per unit of fuel over distance travelled". That reduces to energy/distance, or: ( kg × m ) / s^2. That doesn't have a unit applied to it, though it works out to mass times acceleration. So, as noted, "Liters of petrol" (or diesel) are actually a shorthand for "energy contained in a liter of fuel". There's an imperial unit for this, the barrel of oil equivalent, "barreloil" in GNU Units. This is equivalent to: • 6.12 billion Joules • 1.70 MWh • 5.8 million BTU • 0.000107 Hiroshima bombs • 0.21 tons of coal • 3.82e+28 electron volts • 1.46 million kilocalories (food calories) So, a 30 MPG car (7.8 liters/100km) is burning 0.049 barrels of oil per 100km, or roughly 83.4 kWh/100km. I happened to catch a recent video Robert Llewellyn posted to G+ on the VW e-Golf, an all-electric version of the Golf (previously the Rabbit in the US)[1]. He mentioned getting approximately 15 kWh/100km driving performance, with the manufacturer claiming efficiency as high as 12 kWh/100km. This means that, overall, the electric vehicle is a lot more efficient than a typical internal-combustion vehicle. Checking on Edmunds.com[2], the 2014 Golf Diesel 2.0L 6 speed manual is rated for 30/42 mpg -- the eGolf uses nearly 7x less energy (6.95x) on the road. Mind that generating and transmission losses run around 30%, so the net efficiency gain is only 4.87x, but that's still a lot more efficient than an oil-powered vehicle. The eGolf does offer far less range than the oil-powered version, however, as a result of the much lower energy storage density offered by batteries over liquid fuels. _______________________________ Notes: 1. https://plus.google.com/114018232303831249060/posts/SLeXV2o4jzF https://plus.google.com/114018232303831249060/posts/SLeXV2o4... http://www.youtube.com/watch?v=C2ZDVfa3x7Y http://www.youtube.com/watch?v=C2ZDVfa3x7Y 2. http://www.edmunds.com/volkswagen/golf/2014/?sub=diesel#specs-pod-anchor http://www.edmunds.com/volkswagen/golf/2014/?sub=diesel#spec...
dnautics 13y ago
> Fuel consumption is a better measure of a vehicles performance because it is a linear relationship with the fuel used, as opposed to fuel economy which has an inherent reciprocal distortion. Fuel economy is a better measure of travel potential because it is a linear relationship with the distance you can go, as opposed to fuel consumption, which has an inherent reciprocal distortion.
- RaptorJ 13y ago
  An interesting switch, but who cares? There are gas stations everywhere.
  - username223 13y ago
    Everywhere? Try driving across Nevada or southern Utah.
    - balls187 13y ago
      On the main roads, it looks like there are a decent number of fuel options, within a 50 mile radius. https://www.google.com/maps/search/gas+stations+near+southern+utah/@39.2742659,-111.6872373,7z https://www.google.com/maps/search/gas+stations+near+souther...
      - username223 13y ago
        Realistically, knowing that you have 200 miles of fuel is often useful when driving in that part of the country. There's even one part of I-70 with 100+ miles between gas stations, IIRC.
        balls187 13y ago
        Agreed. My GTi shows an estimation of how much further I can go. I was just pointing out that, in Southern Utah (too which I've never driven), it looks like it's not as barren as was commented on.
analog31 13y ago
I think there are other examples of this, such as price-to-earnings ratio, and focal length of lenses. In those cases, since a denominator can go through zero, the ratio goes through a big discontinuity. And I've seen p/e ratios reported as "n/a" when they should really be represented by a big negative number.
sillysaurus3 13y ago
Also time-per-frame instead of frames-per-second.
- deletes 13y ago
  Frames per seconds is better in almost every way. When you use it to watch movies it is better since a normal human understands seconds but not what a frame is. In gaming, frame times usually vary quite a lot, and the average, which is fps, is the important part.
  - ygra 13y ago
    When doing real-time graphics and looking at a profiler, it does help to know that you want to have less than 16 ms of calculations per frame. But agreed, in most other respects fps is nicer.
werdnapk 13y ago
Canada pretty much uses X litres/100km nowadays recently replacing mpg. The lower the litres, the more efficient the vehicle. It actually was fairly easy to become accustomed to these new metrics.
- burke 13y ago
  It's an extremely convenient metric. L/100km is roughly equivalent to L/h at highway speeds, and you can figure Litres to destination when you see a road sign by multiplying L/100km by the hundreds place. With gas stations being upwards of a hundred kilometres apart in the prairies and northern ontario, especially at night, it's nice that this is so easy.
  - blahedo 13y ago
    Ah, good point: so that means we should be using Gallons per 60 miles! :)
dredmorbius 13y ago
MPG is useful for range estimates: if you've got n gallons, you can travel m miles. GPM is useful for budget estimates: if you have a trip of m miles, you'll need n gallons of fuel, with some given cost. Or you can use a tool such as GNU units which handles reciprocal conversions with ease and aplomb.
- gamegoblin 13y ago
  Never used GNU units, but Wolfram Alpha will also do this trivially [0]. [0] http://www.wolframalpha.com/input/?i=16+mpg+in+barrels%2Ffurlong http://www.wolframalpha.com/input/?i=16+mpg+in+barrels%2Ffur...
  - return0 13y ago
    and google
  - cbr 13y ago
    As will google: https://www.google.com/search?q=16+mpg+in+barrels%2Ffurlong https://www.google.com/search?q=16+mpg+in+barrels%2Ffurlong
  - dredmorbius 13y ago
    GNU units works offline ;-)
- astrodust 13y ago
  I don't know how a/b is somehow magically easier than b/a. They're both involving division. Most cars outside of the US report L/100km. For example, if you have to go 250km and you're running at 8L/100km then you need 2.5x8 = 20L. If your tank holds 50L, then you'll need nearly a half tank. If you want to know how far you can get on a half tank, say 25L, then divide that by 8 to get roughly 6, or 600km.
- bigbugbag 13y ago
  AFAIK, MPG is not used anywhere in the world but in one country where people don't even grasp the beginning of it. MPG is actually quite useless to do range estimates. If you got n gallons you could possibly maybe travel more or less m miles depending on a variety of factors unrelated to the vehicle fuel efficiency such as speed, terrain, open or closed windows, driving with A/C on, and much more. There are a few mythbusters episodes dedicated to the myths of fuel efficiency/consumption. GPM doesn't seem that useful for budget estimates either as this depends on the fuel price, well maybe it is in the US where oil price is maintained artificially low but in other parts of the world you have a hard time guessing the price of oil 6 months from now or at the next gas station. I've seen price fluctuation of 0.8$ per gallon from a week to the next and 1.5$ per gallon from a gas station to the next in the same city. GPM is only useful to compare vehicles between them, which is something people in position of power in the USA do not want.
taeric 13y ago
Citations, or you are just promoting your opinion. I'm also curious about the numbers showing that 55mph is the most efficient. I do not particularly dispute that this is the case for the majority of vehicles. I am curious as to whether or not this is as it has to be, or because that is the way vehicles are built in the US. That is, could you do better with different high speed gearing?
- squeakynick 13y ago
  There's a wealth of information here http://www.fueleconomy.gov/ http://www.fueleconomy.gov/
- exue 13y ago
  It's not always the most efficient for every car, but there are lots of charts showing this tradeoff. Some cars are more efficient at 65 or 70 than 55 (97 Celica), but in modern gas cars, 40-50mph is where you hit a peak, and you start getting worse from there. 40mph on the highway is painfully slow though, so people generally pick an optimal point higher than that. (One inconsistency I've found is how the Motor Trend test below sees a very high peak at about 40MPH and a decline after, while other sources show a flatter peak). And 55MPH seems more like a magic number the author picked, but not a bad one if we don't know the model. Generally cars are geared so the engine begins entering its optimal / most efficient RPM range where it can produce more torque in top gear around 40-50mph, and rolling resistance and wind drag (which is a cube function of speed) contribute to make mileage worse after that. Usually the speed at which you first shift into top gear and cruise comfortably is close to optimal. Efficiency doesn't start dropping for a while because of the cubic nature of drag, and the engine sometimes is more efficient at higher RPMs for a bit (see nols' post on manufacturer optimization). Fun fact: At top speed (254mph), a Bugatti Veyron will use its 26 gallon tank in 12 minutes. An older chart: http://upload.wikimedia.org/wikipedia/commons/a/a9/Fuel_economy_vs_speed_1997.png http://upload.wikimedia.org/wikipedia/commons/a/a9/Fuel_econ... A newer chart: http://blog.automatic.com/wp-content/uploads/2013/12/mpg-vs-speed-all.png http://blog.automatic.com/wp-content/uploads/2013/12/mpg-vs-... Motor Trend chart: http://image.motortrend.com/f/roadtests/sedans/1208_40_mpg_compact_sedan_comparison/38489659/40-MPG-comparison-constant-speed-chart.jpg http://image.motortrend.com/f/roadtests/sedans/1208_40_mpg_c... http://www.motortrend.com/roadtests/sedans/1208_40_mpg_compact_sedan_comparison/ http://www.motortrend.com/roadtests/sedans/1208_40_mpg_compa... (One thing that bothered me is how the Civics in the last two charts were so different)
pbreit 13y ago
Either I did not understand the post or it is totally uncompelling. MPG makes much more sense to me. Gallons per 100 miles (or whatever) sounds very clumsy.
- sopooneo 13y ago
  A typical person drives a fixed number of miles per month, rather than using a fixed number of gallons of gas. So one advantage of "Gallons / 100 miles" is that it requires one fewer steps of arithmetic to figure out how much gas you need every month.
  - lotharbot 13y ago
    On the other hand, a typical person driving between Denver and New Orleans knows they just put 14 gallons in the tank and would probably like to figure out how far until their next required fuel stop. Different use cases mean different natural measurements.
    - Vik1ng 13y ago
      It's not that bad then either. If you have 3 gallons per 100 miles, then 3 fits into 14 a bit less than 5 times so you will get like 470 miles. I mean it's not going to be exact anyway depending on the conditions.
      - lotharbot 13y ago
        right, I'm not saying one measurement is "bad" and the other is "good". Just that one is more natural for specific conditions. Something I learned in grad school (applied mathematics): a lot of people care about metric because the conversions are easy. But a lot of systems are better measured in units that come from the system itself. Traffic flow may be best measured, not in feet or meters, but in multiples of the average car length. If you get too caught up in trying to force everything into metric just because it's usually better, you can end up creating extra work for yourself in circumstances where the system is better measured using itself. Likewise, if you insist on gpm because it's "better" than mpg, you make some calculations a little easier and others a little harder.
amluto 13y ago
I was hoping that an article about careful use of measurement units wouldn't contain a blatant mathematical error. The article calculates, presumably correctly, that driving 70mph costs $3.66/hr more than driving 55mph in some reference car. It concludes that driving 70mph is worthwhile if getting to your destination an hour early is worth more than $3.66. This is completely wrong. Driving 70mph for an hour gets you 15 miles farther than driving 55mph for an hour. Doing that costs you less than $3.66, since you're driving for less time. It also doesn't save you anywhere near an hour.
- vlasev 13y ago
  I think that's as good of an example as any provided by the author as why the US should do away with the mpg measurement. If even the careful author can make a mistake like that, then think about the average person.
pash 13y ago
This topic came up on HN a couple of years ago [0], and I posted a comment that was well received. So here it is again, edited for the present context and updated with my recent thoughts: People do seem to misinterpret MPG ratings. A study published in Science in 2008 [1] found that participants consistently overvalued vehicles with high MPG ratings. They assigned values linear in MPG rather than linear in its inverse. The study's authors told participants to "assume you drive 10,000 miles per year for work, and this total amount cannot be changed." The participants were then asked to come up with values for vehicles of varying fuel-efficiencies. That is just the sort of optimization problem people face when choosing which car to buy, and apparently a fuel-efficiency metric that puts the amount of fuel in the numerator makes the problem easier to solve because expenditure is proportional to the amount of fuel burned, at least when distance driven is taken as given. But in reading many of the words expended on this topic over the last couple of years, my lasting impression is that this a lot of hullabaloo about the wrong problem. It's the lack of attention paid to the "miles" part of the equation that most needs fixing. If the goal is to reduce carbon emissions (or any of the other negative externalities of driving), then taking distance driven as fixed frames the problem in a way that obscures the real solution: we should be encouraging people to drive less. Yes, reordering your daily life to drive fewer miles is more disruptive than simply buying a car that goes farther on a gallon of gas. And, granted, once you've chosen your style of life, minimizing the amount of gas you burn as you go about your daily routine is the thing to do (even if your optimization problem is a purely financial one). All the same, it's ludicrous to ignore the basic inefficiency of the suburban lifestyle that predominates in America while we wait for automotive engineers to come up with clever solutions to pricey gas and to carbon emissions that are twice as high per capita as in similarly wealthy countries. Elon Musk isn't going to save us all by himself. Surely living closer to where you work, using mass transit, cycling, and walking more must be part of the solution as well. ... So maybe houses and apartments should come with a "miles per day" rating suggesting how far you'd travel getting to and from shops, restaurants, entertainment venues, and your place of work every day you live there. ... 0. https://news.ycombinator.com/item?id=4020885 https://news.ycombinator.com/item?id=4020885 1. http://www.sciencemag.org/content/320/5883/1593.summary http://www.sciencemag.org/content/320/5883/1593.summary (paywalled); http://nsmn1.uh.edu/dgraur/niv/theMPGIllusion.pdf http://nsmn1.uh.edu/dgraur/niv/theMPGIllusion.pdf [PDF]
danbruc 13y ago
Next: Why liters per 100 kilometers is better than gallons per mile.
- pash 13y ago
  See https://news.ycombinator.com/item?id=4020885 https://news.ycombinator.com/item?id=4020885 :P
  - notduncansmith 13y ago
    Well played, sir.
- Vik1ng 13y ago
  It's the only one that doesn't make my brain hurt xd Every time I'm confronted with MPG I first have to think what is better and when I calculate I always confuse M/G and G/M. While always having that 100km distance in mind makes it a lot easier. I'm not suddenly going to think about 100l.
- thomasahle 13y ago
  With most cars today driving more than 20km/L, it would perhaps make sense to change to L/1000km already.
  - ntaso 13y ago
    On paper maybe, but in reality, most cars still consume between 5-10 liters per 100km and more, if you have the horse power.
leccine 13y ago
I guess this is why l / 100km is used in the EU instead of km / l.
- Ecio78 13y ago
  Actually in Italy I've always seen km per liter for years and years, let's say all the 80s and 90s ("my car runs 14 km per liter, and yours?" "mine is crap, I can't do more than 10km with a liter of gasoline" and same for auto magazines). I think only recently (new century?) they started using more the "liters per 100km" approach
shmerl 13y ago
How about switching to the metric system altogether?
SixSigma 13y ago
The United Nations creating a uniform system of regulations, WP.29 was established on June 1952, for vehicle design to facilitate international trade. http://en.wikipedia.org/wiki/World_Forum_for_Harmonization_of_Vehicle_Regulations http://en.wikipedia.org/wiki/World_Forum_for_Harmonization_o... As of 2012, the participants to the 1958 Agreement include 52 countries including all of Europe, China, Russia, Japan, Australia. Part of the regulations is a common specification for fuel use - litres per 100km. There is one significant non-signatory to the agreement.
- jaggederest 13y ago
  Could even use gallons per 250 miles as a fair approximation in US-style units, and I think it's a more practical measure than gallons per mile since it indicates perhaps a week of moderately heavy driving.
  - twic 13y ago
    Gallons per 280 miles is even closer. And since there are seven days in a week, you can weakly justify that as 40 miles a day every day of the week.
- bigbugbag 13y ago
  Could it be the very country that is among the three that refuses to use the metric system with myanmar and liberia ? http://deandecrease.wordpress.com/2012/11/22/we-dont-need-no-stinkin-metric-system/ http://deandecrease.wordpress.com/2012/11/22/we-dont-need-no...
return0 13y ago
Well, "liters per kilometer" would be even better.
- bigbugbag 13y ago
  I disagree. First, the human mind is not that good at dealing with decimal and numbers between 0 and 1. My car is rated 4.5l / 100 km, which is easier to deal with than 0.045l / km. For a simple calculation, say 5km, what the easiest 0.045 * 5 or (4.5/10)/2 ? Secondly, a car is designed to be driven over distance, so making measurement on only one kilometer wouldn't be representative of real world usage.
  - return0 13y ago
    agree, anything but gallons or miles. L/Km is the standard in europe anyway.
bigbugbag 13y ago
From a European point of view it seem obvious that the gallons per mile trick is a small but effective cog in the larger US automobile conspiracy going on for decades. From the destruction of public transportation[1] to the many billions of dollars bailouts and current tesla battle this conspiracy is quite an obvious and documented one. [1] https://en.wikipedia.org/wiki/General_Motors_Streetcar_Conspiracy https://en.wikipedia.org/wiki/General_Motors_Streetcar_Consp...

115 points by squeakynick 13y ago

deleted 13y ago

[deleted]

anigbrowl 13y ago

I'll settle for reciprocal distortion if more people would switch to the metric system.

dredmorbius 13y ago
Most metric countries report liters/100km, solving both problems at the same time.
- ProblemFactory 13y ago
  An useless but fun observation: you can do the litres/100km division and arrive at units of area. A family car might have a fuel consumption of 0.05 mm^2. This is the cross-section of the imaginary pipe of fuel along the highway that a car with no fuel tank would have to pick up as it travels.
  - keule 13y ago
    https://what-if.xkcd.com/11/ https://what-if.xkcd.com/11/
  - dredmorbius 13y ago
    Actually, what you're arriving at is energy use per unit distance. The unit of area result you're getting (correcting from original content) is a misuse of units: "liters/100km" is more properly "liters of fuel/100km", where liters-of-fuel is a shorthand for quantity (that is, mass) of fuel used per 100km -- it's more convenient to measure fluid volume than mass (though you'll find that aircraft will measure fuel by weight). And that is still a shorthand for "energy contained per unit of fuel over distance travelled". That reduces to energy/distance, or: ( kg × m ) / s^2. That doesn't have a unit applied to it, though it works out to mass times acceleration. So, as noted, "Liters of petrol" (or diesel) are actually a shorthand for "energy contained in a liter of fuel". There's an imperial unit for this, the barrel of oil equivalent, "barreloil" in GNU Units. This is equivalent to: • 6.12 billion Joules • 1.70 MWh • 5.8 million BTU • 0.000107 Hiroshima bombs • 0.21 tons of coal • 3.82e+28 electron volts • 1.46 million kilocalories (food calories) So, a 30 MPG car (7.8 liters/100km) is burning 0.049 barrels of oil per 100km, or roughly 83.4 kWh/100km. I happened to catch a recent video Robert Llewellyn posted to G+ on the VW e-Golf, an all-electric version of the Golf (previously the Rabbit in the US)[1]. He mentioned getting approximately 15 kWh/100km driving performance, with the manufacturer claiming efficiency as high as 12 kWh/100km. This means that, overall, the electric vehicle is a lot more efficient than a typical internal-combustion vehicle. Checking on Edmunds.com[2], the 2014 Golf Diesel 2.0L 6 speed manual is rated for 30/42 mpg -- the eGolf uses nearly 7x less energy (6.95x) on the road. Mind that generating and transmission losses run around 30%, so the net efficiency gain is only 4.87x, but that's still a lot more efficient than an oil-powered vehicle. The eGolf does offer far less range than the oil-powered version, however, as a result of the much lower energy storage density offered by batteries over liquid fuels. _______________________________ Notes: 1. https://plus.google.com/114018232303831249060/posts/SLeXV2o4jzF https://plus.google.com/114018232303831249060/posts/SLeXV2o4... http://www.youtube.com/watch?v=C2ZDVfa3x7Y http://www.youtube.com/watch?v=C2ZDVfa3x7Y 2. http://www.edmunds.com/volkswagen/golf/2014/?sub=diesel#specs-pod-anchor http://www.edmunds.com/volkswagen/golf/2014/?sub=diesel#spec...

dnautics 13y ago

> Fuel consumption is a better measure of a vehicles performance because it is a linear relationship with the fuel used, as opposed to fuel economy which has an inherent reciprocal distortion. Fuel economy is a better measure of travel potential because it is a linear relationship with the distance you can go, as opposed to fuel consumption, which has an inherent reciprocal distortion.

RaptorJ 13y ago
An interesting switch, but who cares? There are gas stations everywhere.
- username223 13y ago
  Everywhere? Try driving across Nevada or southern Utah.
  - balls187 13y ago
    On the main roads, it looks like there are a decent number of fuel options, within a 50 mile radius. https://www.google.com/maps/search/gas+stations+near+southern+utah/@39.2742659,-111.6872373,7z https://www.google.com/maps/search/gas+stations+near+souther...
    - username223 13y ago
      Realistically, knowing that you have 200 miles of fuel is often useful when driving in that part of the country. There's even one part of I-70 with 100+ miles between gas stations, IIRC.
      - balls187 13y ago
        Agreed. My GTi shows an estimation of how much further I can go. I was just pointing out that, in Southern Utah (too which I've never driven), it looks like it's not as barren as was commented on.

analog31 13y ago

I think there are other examples of this, such as price-to-earnings ratio, and focal length of lenses. In those cases, since a denominator can go through zero, the ratio goes through a big discontinuity. And I've seen p/e ratios reported as "n/a" when they should really be represented by a big negative number.

sillysaurus3 13y ago

Also time-per-frame instead of frames-per-second.

deletes 13y ago
Frames per seconds is better in almost every way. When you use it to watch movies it is better since a normal human understands seconds but not what a frame is. In gaming, frame times usually vary quite a lot, and the average, which is fps, is the important part.
- ygra 13y ago
  When doing real-time graphics and looking at a profiler, it does help to know that you want to have less than 16 ms of calculations per frame. But agreed, in most other respects fps is nicer.

werdnapk 13y ago

Canada pretty much uses X litres/100km nowadays recently replacing mpg. The lower the litres, the more efficient the vehicle. It actually was fairly easy to become accustomed to these new metrics.

burke 13y ago
It's an extremely convenient metric. L/100km is roughly equivalent to L/h at highway speeds, and you can figure Litres to destination when you see a road sign by multiplying L/100km by the hundreds place. With gas stations being upwards of a hundred kilometres apart in the prairies and northern ontario, especially at night, it's nice that this is so easy.
- blahedo 13y ago
  Ah, good point: so that means we should be using Gallons per 60 miles! :)

dredmorbius 13y ago

MPG is useful for range estimates: if you've got n gallons, you can travel m miles. GPM is useful for budget estimates: if you have a trip of m miles, you'll need n gallons of fuel, with some given cost. Or you can use a tool such as GNU units which handles reciprocal conversions with ease and aplomb.

gamegoblin 13y ago
Never used GNU units, but Wolfram Alpha will also do this trivially [0]. [0] http://www.wolframalpha.com/input/?i=16+mpg+in+barrels%2Ffurlong http://www.wolframalpha.com/input/?i=16+mpg+in+barrels%2Ffur...
- return0 13y ago
  and google
- cbr 13y ago
  As will google: https://www.google.com/search?q=16+mpg+in+barrels%2Ffurlong https://www.google.com/search?q=16+mpg+in+barrels%2Ffurlong
- dredmorbius 13y ago
  GNU units works offline ;-)
astrodust 13y ago
I don't know how a/b is somehow magically easier than b/a. They're both involving division. Most cars outside of the US report L/100km. For example, if you have to go 250km and you're running at 8L/100km then you need 2.5x8 = 20L. If your tank holds 50L, then you'll need nearly a half tank. If you want to know how far you can get on a half tank, say 25L, then divide that by 8 to get roughly 6, or 600km.
bigbugbag 13y ago
AFAIK, MPG is not used anywhere in the world but in one country where people don't even grasp the beginning of it. MPG is actually quite useless to do range estimates. If you got n gallons you could possibly maybe travel more or less m miles depending on a variety of factors unrelated to the vehicle fuel efficiency such as speed, terrain, open or closed windows, driving with A/C on, and much more. There are a few mythbusters episodes dedicated to the myths of fuel efficiency/consumption. GPM doesn't seem that useful for budget estimates either as this depends on the fuel price, well maybe it is in the US where oil price is maintained artificially low but in other parts of the world you have a hard time guessing the price of oil 6 months from now or at the next gas station. I've seen price fluctuation of 0.8$ per gallon from a week to the next and 1.5$ per gallon from a gas station to the next in the same city. GPM is only useful to compare vehicles between them, which is something people in position of power in the USA do not want.

taeric 13y ago

Citations, or you are just promoting your opinion. I'm also curious about the numbers showing that 55mph is the most efficient. I do not particularly dispute that this is the case for the majority of vehicles. I am curious as to whether or not this is as it has to be, or because that is the way vehicles are built in the US. That is, could you do better with different high speed gearing?

squeakynick 13y ago
There's a wealth of information here http://www.fueleconomy.gov/ http://www.fueleconomy.gov/
exue 13y ago
It's not always the most efficient for every car, but there are lots of charts showing this tradeoff. Some cars are more efficient at 65 or 70 than 55 (97 Celica), but in modern gas cars, 40-50mph is where you hit a peak, and you start getting worse from there. 40mph on the highway is painfully slow though, so people generally pick an optimal point higher than that. (One inconsistency I've found is how the Motor Trend test below sees a very high peak at about 40MPH and a decline after, while other sources show a flatter peak). And 55MPH seems more like a magic number the author picked, but not a bad one if we don't know the model. Generally cars are geared so the engine begins entering its optimal / most efficient RPM range where it can produce more torque in top gear around 40-50mph, and rolling resistance and wind drag (which is a cube function of speed) contribute to make mileage worse after that. Usually the speed at which you first shift into top gear and cruise comfortably is close to optimal. Efficiency doesn't start dropping for a while because of the cubic nature of drag, and the engine sometimes is more efficient at higher RPMs for a bit (see nols' post on manufacturer optimization). Fun fact: At top speed (254mph), a Bugatti Veyron will use its 26 gallon tank in 12 minutes. An older chart: http://upload.wikimedia.org/wikipedia/commons/a/a9/Fuel_economy_vs_speed_1997.png http://upload.wikimedia.org/wikipedia/commons/a/a9/Fuel_econ... A newer chart: http://blog.automatic.com/wp-content/uploads/2013/12/mpg-vs-speed-all.png http://blog.automatic.com/wp-content/uploads/2013/12/mpg-vs-... Motor Trend chart: http://image.motortrend.com/f/roadtests/sedans/1208_40_mpg_compact_sedan_comparison/38489659/40-MPG-comparison-constant-speed-chart.jpg http://image.motortrend.com/f/roadtests/sedans/1208_40_mpg_c... http://www.motortrend.com/roadtests/sedans/1208_40_mpg_compact_sedan_comparison/ http://www.motortrend.com/roadtests/sedans/1208_40_mpg_compa... (One thing that bothered me is how the Civics in the last two charts were so different)

pbreit 13y ago

Either I did not understand the post or it is totally uncompelling. MPG makes much more sense to me. Gallons per 100 miles (or whatever) sounds very clumsy.

sopooneo 13y ago
A typical person drives a fixed number of miles per month, rather than using a fixed number of gallons of gas. So one advantage of "Gallons / 100 miles" is that it requires one fewer steps of arithmetic to figure out how much gas you need every month.
- lotharbot 13y ago
  On the other hand, a typical person driving between Denver and New Orleans knows they just put 14 gallons in the tank and would probably like to figure out how far until their next required fuel stop. Different use cases mean different natural measurements.
  - Vik1ng 13y ago
    It's not that bad then either. If you have 3 gallons per 100 miles, then 3 fits into 14 a bit less than 5 times so you will get like 470 miles. I mean it's not going to be exact anyway depending on the conditions.
    - lotharbot 13y ago
      right, I'm not saying one measurement is "bad" and the other is "good". Just that one is more natural for specific conditions. Something I learned in grad school (applied mathematics): a lot of people care about metric because the conversions are easy. But a lot of systems are better measured in units that come from the system itself. Traffic flow may be best measured, not in feet or meters, but in multiples of the average car length. If you get too caught up in trying to force everything into metric just because it's usually better, you can end up creating extra work for yourself in circumstances where the system is better measured using itself. Likewise, if you insist on gpm because it's "better" than mpg, you make some calculations a little easier and others a little harder.

amluto 13y ago

I was hoping that an article about careful use of measurement units wouldn't contain a blatant mathematical error. The article calculates, presumably correctly, that driving 70mph costs $3.66/hr more than driving 55mph in some reference car. It concludes that driving 70mph is worthwhile if getting to your destination an hour early is worth more than $3.66. This is completely wrong. Driving 70mph for an hour gets you 15 miles farther than driving 55mph for an hour. Doing that costs you less than $3.66, since you're driving for less time. It also doesn't save you anywhere near an hour.

vlasev 13y ago
I think that's as good of an example as any provided by the author as why the US should do away with the mpg measurement. If even the careful author can make a mistake like that, then think about the average person.

pash 13y ago

This topic came up on HN a couple of years ago [0], and I posted a comment that was well received. So here it is again, edited for the present context and updated with my recent thoughts: People do seem to misinterpret MPG ratings. A study published in Science in 2008 [1] found that participants consistently overvalued vehicles with high MPG ratings. They assigned values linear in MPG rather than linear in its inverse. The study's authors told participants to "assume you drive 10,000 miles per year for work, and this total amount cannot be changed." The participants were then asked to come up with values for vehicles of varying fuel-efficiencies. That is just the sort of optimization problem people face when choosing which car to buy, and apparently a fuel-efficiency metric that puts the amount of fuel in the numerator makes the problem easier to solve because expenditure is proportional to the amount of fuel burned, at least when distance driven is taken as given. But in reading many of the words expended on this topic over the last couple of years, my lasting impression is that this a lot of hullabaloo about the wrong problem. It's the lack of attention paid to the "miles" part of the equation that most needs fixing. If the goal is to reduce carbon emissions (or any of the other negative externalities of driving), then taking distance driven as fixed frames the problem in a way that obscures the real solution: we should be encouraging people to drive less. Yes, reordering your daily life to drive fewer miles is more disruptive than simply buying a car that goes farther on a gallon of gas. And, granted, once you've chosen your style of life, minimizing the amount of gas you burn as you go about your daily routine is the thing to do (even if your optimization problem is a purely financial one). All the same, it's ludicrous to ignore the basic inefficiency of the suburban lifestyle that predominates in America while we wait for automotive engineers to come up with clever solutions to pricey gas and to carbon emissions that are twice as high per capita as in similarly wealthy countries. Elon Musk isn't going to save us all by himself. Surely living closer to where you work, using mass transit, cycling, and walking more must be part of the solution as well. ... So maybe houses and apartments should come with a "miles per day" rating suggesting how far you'd travel getting to and from shops, restaurants, entertainment venues, and your place of work every day you live there. ... 0. https://news.ycombinator.com/item?id=4020885 https://news.ycombinator.com/item?id=4020885 1. http://www.sciencemag.org/content/320/5883/1593.summary http://www.sciencemag.org/content/320/5883/1593.summary (paywalled); http://nsmn1.uh.edu/dgraur/niv/theMPGIllusion.pdf http://nsmn1.uh.edu/dgraur/niv/theMPGIllusion.pdf [PDF]

danbruc 13y ago

Next: Why liters per 100 kilometers is better than gallons per mile.

pash 13y ago
See https://news.ycombinator.com/item?id=4020885 https://news.ycombinator.com/item?id=4020885 :P
- notduncansmith 13y ago
  Well played, sir.
Vik1ng 13y ago
It's the only one that doesn't make my brain hurt xd Every time I'm confronted with MPG I first have to think what is better and when I calculate I always confuse M/G and G/M. While always having that 100km distance in mind makes it a lot easier. I'm not suddenly going to think about 100l.
thomasahle 13y ago
With most cars today driving more than 20km/L, it would perhaps make sense to change to L/1000km already.
- ntaso 13y ago
  On paper maybe, but in reality, most cars still consume between 5-10 liters per 100km and more, if you have the horse power.

leccine 13y ago

I guess this is why l / 100km is used in the EU instead of km / l.

Ecio78 13y ago
Actually in Italy I've always seen km per liter for years and years, let's say all the 80s and 90s ("my car runs 14 km per liter, and yours?" "mine is crap, I can't do more than 10km with a liter of gasoline" and same for auto magazines). I think only recently (new century?) they started using more the "liters per 100km" approach

shmerl 13y ago

How about switching to the metric system altogether?

SixSigma 13y ago

The United Nations creating a uniform system of regulations, WP.29 was established on June 1952, for vehicle design to facilitate international trade. http://en.wikipedia.org/wiki/World_Forum_for_Harmonization_of_Vehicle_Regulations http://en.wikipedia.org/wiki/World_Forum_for_Harmonization_o... As of 2012, the participants to the 1958 Agreement include 52 countries including all of Europe, China, Russia, Japan, Australia. Part of the regulations is a common specification for fuel use - litres per 100km. There is one significant non-signatory to the agreement.

jaggederest 13y ago
Could even use gallons per 250 miles as a fair approximation in US-style units, and I think it's a more practical measure than gallons per mile since it indicates perhaps a week of moderately heavy driving.
- twic 13y ago
  Gallons per 280 miles is even closer. And since there are seven days in a week, you can weakly justify that as 40 miles a day every day of the week.
bigbugbag 13y ago
Could it be the very country that is among the three that refuses to use the metric system with myanmar and liberia ? http://deandecrease.wordpress.com/2012/11/22/we-dont-need-no-stinkin-metric-system/ http://deandecrease.wordpress.com/2012/11/22/we-dont-need-no...

return0 13y ago

Well, "liters per kilometer" would be even better.

bigbugbag 13y ago
I disagree. First, the human mind is not that good at dealing with decimal and numbers between 0 and 1. My car is rated 4.5l / 100 km, which is easier to deal with than 0.045l / km. For a simple calculation, say 5km, what the easiest 0.045 * 5 or (4.5/10)/2 ? Secondly, a car is designed to be driven over distance, so making measurement on only one kilometer wouldn't be representative of real world usage.
- return0 13y ago
  agree, anything but gallons or miles. L/Km is the standard in europe anyway.

bigbugbag 13y ago

From a European point of view it seem obvious that the gallons per mile trick is a small but effective cog in the larger US automobile conspiracy going on for decades. From the destruction of public transportation[1] to the many billions of dollars bailouts and current tesla battle this conspiracy is quite an obvious and documented one. [1] https://en.wikipedia.org/wiki/General_Motors_Streetcar_Conspiracy https://en.wikipedia.org/wiki/General_Motors_Streetcar_Consp...