5 ms·

It’s even more confusing than that. They use the British system prior to imperial standardisation, so one has both US customary and imperial units of inch etc m

It’s even more confusing than that. They use the British system prior to imperial standardisation, so one has both US customary and imperial units of inch etc measurements. Which is why volumes and amount of sub units are different between the two.

noneeeed 6y ago
It gets particularly confusing when people start comparing mpg for US and British cars, as the two gallons are not the same, I've seen so many arguments online where people are flat out comparing differenteasures and assuming they were the same.
- masklinn 6y ago
  > It gets particularly confusing when people start comparing mpg for US and British cars, as the two gallons are not the same TBF mpg is a terrible unit to compare car efficiency regardless of which gallon you use. It's only more convenient to evaluate the range you have on a tank.
  - noneeeed 6y ago
    I'm obviously missing something. Why would distance travelled on a unit of fuel be a bad way to measure the efficiency of a car? I know that such measures are always imprecise due to different driving styles/conditions etc. So what is a better way to measure the efficiency of a car?
    - masklinn 6y ago
      > I'm obviously missing something. Why would distance travelled on a unit of fuel be a bad way to measure the efficiency of a car? Because it has a sublinear relationship to absolute efficiency: an 11mpg car is significantly more efficient than a 10mpg car, while a 41mpg car much less so in relation to a 40mpg car: both provide an extra mile per gallon, which translates into a 10% and 2.5% increase in efficiency respectively. > So what is a better way to measure the efficiency of a car? gpm (aka L/100km which is what's used in most metric countries). It's less convenient when you know how much fuel you have and want to know what distance you can expect, but it's way more convenient to compare efficiencies, as well as to calculate how much fuel you'll need for a given trip: the examples above are 21.4L/100, 23.5L/100, 5.7 L/100km and 5.9 L/100km. From this, it's obvious that the absolute gain in efficiency is much greater going from 10 to 11 (2L/100km economy) than from 40 to 41 (0.2L/100km)
      - addicted 6y ago
        Well, even if the better unit is liters per 100km, you’d still face the same problems because the UK/US would be talking about gallons per x miles where they would still face the issue the poster described of differing units of gallons.
        masklinn 6y ago
        The point I was rather explicitly (if you go read the original comment) making is that mpgs are bad for comparing fuel economies regardless of differences in what is meant by "gallon", because it's intrinsically not a good unit for comparing fuel efficiencies.
      - noneeeed 6y ago
        Ah, I see, that makes a lot of sense, thanks for the explanation.
    - bjterry 6y ago
      I tried to explain this, but it turns out to be kind of hard, so instead I found a reference for you: https://www.bunniestudios.com/blog/?p=257 https://www.bunniestudios.com/blog/?p=257 Basically, it makes it easier to compare fuel economy differences at different points in the scale because they become subtraction problems instead of division problems. This is because usually the amount you are going to drive is fixed, and MPG puts the miles in the numerator, while "gpm" puts the fixed miles in the denominator, I think.
      - tridentlead 6y ago
        Only if you don't drive for fun. If you do, it's a lot more useful to know how far you can get on a tank.
        vikramkr 6y ago
        The best way I've heard it phrased is that europeans 2ant to know much much fuel they need to get to their destination, while americans want to know how far they can go on a tank. Its obviously oversimplified/not relevant/other pedantic nitpicking but it captures the different relationship americans in general have with driving and their cars
        njarboe 6y ago
        Maybe this is because most people with cars in America (especially in the 1950's) were not worried much about the price of gas. Europe had much higher gas prices and was much poorer, so how much it would cost (how much gas would be used to get somewhere) was more important for planning than knowing when they would need to fill up again.
        masklinn 6y ago
        The original comment was about comparing fuel efficiencies. But to your point: mpg makes it easier to know how far you can travel on a given volume of fuel, L/100 makes it easier to compare fuel efficiencies, and to know how much fuel you would need for a given distance to travel. To me the latter seems generally much more useful than the former. The former provides an easier answer to "how far can I go burning this tank", which feels like a much rarer use-case than "how often will I need to refuel to my destination" (and "can I reach my destination on a single tank") for the general population.
masklinn 6y ago
> They use the British system prior to imperial standardisation, so one has both US customary and imperial units of inch etc measurements. In modern parlance most of these should be identical under the 1959 international yard and pound whose entire point was to unify measurements which previously diverged slightly (as well as define them in metric/SI terms). US "Survey" units are really the pre-intenational versions. The units which were not unified are mostly those of volume, so imperial volumes and US customary volumes differ quite a bit (plus the US has "fluid" and "dry" units): imperial units are the largest, then come US dry units (97% of imperial) followed by US fluid units units (83% of imperial).
- 0BADAC1D 6y ago
  having a unit of weight and a unit of volume share the same name is unspeakably stupid. why? WHY?
  - masklinn 6y ago
    Apparently it's because a fluid ounce of water is about an (avoirdupois) ounce so, you know, super helpful reminder! Like the pound being a unit of currency, a unit of mass, a unit of force, a unit of energy and a unit of torque (especially the latter two, who doesn't love having foot-pound and foot-pound being completely different things?)
    - fanf2 6y ago
      An imperial fluid ounce was defined (metric style) to be exactly the volume of an ounce of water. The reason for the change in number of fl.oz. in a pint was to make a gallon weigh 10 lb.
    - ghaff 6y ago
      If you're in an engineering discipline that does calculations involving mass and force, it's probably a good idea to work in metric even if you need to convert the final answer back. At least in my experience, it's way too easy to mess up when using pound force, pound mass, and slugs.

noneeeed 6y ago

It gets particularly confusing when people start comparing mpg for US and British cars, as the two gallons are not the same, I've seen so many arguments online where people are flat out comparing differenteasures and assuming they were the same.

masklinn 6y ago
> It gets particularly confusing when people start comparing mpg for US and British cars, as the two gallons are not the same TBF mpg is a terrible unit to compare car efficiency regardless of which gallon you use. It's only more convenient to evaluate the range you have on a tank.
- noneeeed 6y ago
  I'm obviously missing something. Why would distance travelled on a unit of fuel be a bad way to measure the efficiency of a car? I know that such measures are always imprecise due to different driving styles/conditions etc. So what is a better way to measure the efficiency of a car?
  - masklinn 6y ago
    > I'm obviously missing something. Why would distance travelled on a unit of fuel be a bad way to measure the efficiency of a car? Because it has a sublinear relationship to absolute efficiency: an 11mpg car is significantly more efficient than a 10mpg car, while a 41mpg car much less so in relation to a 40mpg car: both provide an extra mile per gallon, which translates into a 10% and 2.5% increase in efficiency respectively. > So what is a better way to measure the efficiency of a car? gpm (aka L/100km which is what's used in most metric countries). It's less convenient when you know how much fuel you have and want to know what distance you can expect, but it's way more convenient to compare efficiencies, as well as to calculate how much fuel you'll need for a given trip: the examples above are 21.4L/100, 23.5L/100, 5.7 L/100km and 5.9 L/100km. From this, it's obvious that the absolute gain in efficiency is much greater going from 10 to 11 (2L/100km economy) than from 40 to 41 (0.2L/100km)
    - addicted 6y ago
      Well, even if the better unit is liters per 100km, you’d still face the same problems because the UK/US would be talking about gallons per x miles where they would still face the issue the poster described of differing units of gallons.
      - masklinn 6y ago
        The point I was rather explicitly (if you go read the original comment) making is that mpgs are bad for comparing fuel economies regardless of differences in what is meant by "gallon", because it's intrinsically not a good unit for comparing fuel efficiencies.
    - noneeeed 6y ago
      Ah, I see, that makes a lot of sense, thanks for the explanation.
  - bjterry 6y ago
    I tried to explain this, but it turns out to be kind of hard, so instead I found a reference for you: https://www.bunniestudios.com/blog/?p=257 https://www.bunniestudios.com/blog/?p=257 Basically, it makes it easier to compare fuel economy differences at different points in the scale because they become subtraction problems instead of division problems. This is because usually the amount you are going to drive is fixed, and MPG puts the miles in the numerator, while "gpm" puts the fixed miles in the denominator, I think.
    - tridentlead 6y ago
      Only if you don't drive for fun. If you do, it's a lot more useful to know how far you can get on a tank.
      - vikramkr 6y ago
        The best way I've heard it phrased is that europeans 2ant to know much much fuel they need to get to their destination, while americans want to know how far they can go on a tank. Its obviously oversimplified/not relevant/other pedantic nitpicking but it captures the different relationship americans in general have with driving and their cars
        njarboe 6y ago
        Maybe this is because most people with cars in America (especially in the 1950's) were not worried much about the price of gas. Europe had much higher gas prices and was much poorer, so how much it would cost (how much gas would be used to get somewhere) was more important for planning than knowing when they would need to fill up again.
      - masklinn 6y ago
        The original comment was about comparing fuel efficiencies. But to your point: mpg makes it easier to know how far you can travel on a given volume of fuel, L/100 makes it easier to compare fuel efficiencies, and to know how much fuel you would need for a given distance to travel. To me the latter seems generally much more useful than the former. The former provides an easier answer to "how far can I go burning this tank", which feels like a much rarer use-case than "how often will I need to refuel to my destination" (and "can I reach my destination on a single tank") for the general population.

masklinn 6y ago

> They use the British system prior to imperial standardisation, so one has both US customary and imperial units of inch etc measurements. In modern parlance most of these should be identical under the 1959 international yard and pound whose entire point was to unify measurements which previously diverged slightly (as well as define them in metric/SI terms). US "Survey" units are really the pre-intenational versions. The units which were not unified are mostly those of volume, so imperial volumes and US customary volumes differ quite a bit (plus the US has "fluid" and "dry" units): imperial units are the largest, then come US dry units (97% of imperial) followed by US fluid units units (83% of imperial).

0BADAC1D 6y ago
having a unit of weight and a unit of volume share the same name is unspeakably stupid. why? WHY?
- masklinn 6y ago
  Apparently it's because a fluid ounce of water is about an (avoirdupois) ounce so, you know, super helpful reminder! Like the pound being a unit of currency, a unit of mass, a unit of force, a unit of energy and a unit of torque (especially the latter two, who doesn't love having foot-pound and foot-pound being completely different things?)
  - fanf2 6y ago
    An imperial fluid ounce was defined (metric style) to be exactly the volume of an ounce of water. The reason for the change in number of fl.oz. in a pint was to make a gallon weigh 10 lb.
  - ghaff 6y ago
    If you're in an engineering discipline that does calculations involving mass and force, it's probably a good idea to work in metric even if you need to convert the final answer back. At least in my experience, it's way too easy to mess up when using pound force, pound mass, and slugs.