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My car has a realtime mpg output. I can manage 35mpg at 75mph on cruise control. I do not get 40% better economy at 55mph. You know what kills fuel economy? No

by titzer 5y ago

My car has a realtime mpg output. I can manage 35mpg at 75mph on cruise control. I do not get 40% better economy at 55mph. You know what kills fuel economy? Not being on cruise control. After 3000+ highway miles this month, I can definitely say that drivers loitering in the passing lane (there's a reason why they have KEEP RIGHT EXCEPT TO PASS signs) cost enormous amounts of fuel due to the congestion and consequent and avoidable acceleration/deccelaration.

cjohnson318 5y ago
That's very interesting. What kind of car do you drive? Do you know what your fuel efficiency is at 55mph? I would certainly expect there to be some difference between 75mph and 55mph.
- titzer 5y ago
  There is a difference, but it is not 40%. I am usually not in the habit of setting cruise for 55mph on the highway, but I suppose I can try :) Volvo V60, 4 cyl gasoline engine, 2016 model.
  - cjohnson318 5y ago
    For most vehicles, you see fuel efficiency start to take a hit around 55mph. The thing is that the velocity term in the air resistance equation is squared, so at some point your fuel efficiency starts dropping very, very quickly.
- jcims 5y ago
  I ran into a similar issue with a 2003 Honda Accord (v6 manual trans, one of my favorite cars ever). I was spending almost 4 hours a day in the car for a three month job and went through all sorts of experiments to find peak efficiency. If I was on a level section of road, I could get better mileage by driving slower, e.g. 55 vs 75. But when I started to negotiate hills and have to accelerate to maintain speed, the mileage dropped way down because the engine wasn't efficient at generating power at those lower RPMs. And this is where you start to see these unexpected patterns in mileage with gas/diesel engines...they have a very lumpy efficiency curve, and it's regularly more important to stay in a specific RPM range than a specific speed if you want to maximize mileage. Once you start going up to 80-90-100mph physics takes over, of course, but there is a lot of variability in between. At the time I was just thinking fuel prices and it got to the point where I was calculating how much I was saving by driving slower and it was cents per hour. I ultimately landed on 70mph as the sweet spot, and I would regularly get 32mpg.
zip1234 5y ago
1. Most cars actually get worse mileage at higher speeds: Source: https://afdc.energy.gov/data/10312 https://afdc.energy.gov/data/10312 2. Electric cars can go MUCH further at slower speeds than at higher speeds. It is something like 25mph that maximizes range, at least with the Model S and Mach E. 3. Speed governors for cars, though probably highly unpopular amongst drivers, would probably save a lot of fuel.

by titzer 5y ago

cjohnson318 5y ago

That's very interesting. What kind of car do you drive? Do you know what your fuel efficiency is at 55mph? I would certainly expect there to be some difference between 75mph and 55mph.

titzer 5y ago
There is a difference, but it is not 40%. I am usually not in the habit of setting cruise for 55mph on the highway, but I suppose I can try :) Volvo V60, 4 cyl gasoline engine, 2016 model.
- cjohnson318 5y ago
  For most vehicles, you see fuel efficiency start to take a hit around 55mph. The thing is that the velocity term in the air resistance equation is squared, so at some point your fuel efficiency starts dropping very, very quickly.
jcims 5y ago
I ran into a similar issue with a 2003 Honda Accord (v6 manual trans, one of my favorite cars ever). I was spending almost 4 hours a day in the car for a three month job and went through all sorts of experiments to find peak efficiency. If I was on a level section of road, I could get better mileage by driving slower, e.g. 55 vs 75. But when I started to negotiate hills and have to accelerate to maintain speed, the mileage dropped way down because the engine wasn't efficient at generating power at those lower RPMs. And this is where you start to see these unexpected patterns in mileage with gas/diesel engines...they have a very lumpy efficiency curve, and it's regularly more important to stay in a specific RPM range than a specific speed if you want to maximize mileage. Once you start going up to 80-90-100mph physics takes over, of course, but there is a lot of variability in between. At the time I was just thinking fuel prices and it got to the point where I was calculating how much I was saving by driving slower and it was cents per hour. I ultimately landed on 70mph as the sweet spot, and I would regularly get 32mpg.

zip1234 5y ago

1. Most cars actually get worse mileage at higher speeds: Source: https://afdc.energy.gov/data/10312 https://afdc.energy.gov/data/10312 2. Electric cars can go MUCH further at slower speeds than at higher speeds. It is something like 25mph that maximizes range, at least with the Model S and Mach E. 3. Speed governors for cars, though probably highly unpopular amongst drivers, would probably save a lot of fuel.