4 ms·

No, just drive 10 mph slower than usual. Power scales as velocity cubed. So: 80 mph vs 70 mph [1] is a 10/70 increase in speed or 1.14 1.14 ^ 3 = 1.5 or 50%

No, just drive 10 mph slower than usual. Power scales as velocity cubed. So: 80 mph vs 70 mph [1] is a 10/70 increase in speed or 1.14 1.14 ^ 3 = 1.5 or 50% more power required to drive 80 mph vs 70 mph. There are some nuances. ICE efficiency goes up a lot with load. Perhaps you’re not exactly in a v^3 regime. There are fixed power costs. All these and others lessen the impact of doing 80. But the point remains. Slow down. [1] if you dont like my numbers do (vhigh/vlow)^3. The formula doesn't work for very low speeds though (driving 20 in our cars is much more efficient that 10 mph).

tenthirtyam 5y ago
Hmm. But over a fixed distance, the overall energy expenditure scales as v^2 (for v^3 power regime). Double the speed implies octuple (?) the power but only for half the time implies quadruple the energy spend. Not invalidating your point, but just a minor adjustment. :)
- tagoregrtst 5y ago
  I love it :) Never considered that point but, as a rebuttal, consider that trip time vs speed is extremely non linear and stochastic (due to traffic, lights, etc). In my experience you have drive much faster than x% to increase your average speed x% and it doesn't appear to be a linear scaling at all. Of course, you shouldn't be doing 80 in urban roads
nerdponx 5y ago
Subjectively it feels like highway speeds have been going up steadily over the last 10 years. Maybe not. But I don't like that "cruising" speed on US interstates is 70-75. Seems dangerous and wasteful, just to save a few minutes travel time. But then again people also drive on the highways with huge empty pickup trucks all the time, so they probably just don't care about waste.
- danans 5y ago
  > Subjectively it feels like highway speeds have been going up steadily over the last 10 years. Maybe not. But I don't like that "cruising" speed on US interstates is 70-75 Secretly, I hope that range anxiety in EV sticks around a little longer so that people lower their average speeds on the freeway to max out their range. That, coupled with a growing realization that most of the fun with driving is in the peppy acceleration and good handling which is pretty much table stakes for modern EVs across the price spectrum. High sustained top speed in a straight line with an overpowered ICE car is boring and should be buried with the Lamborghinis and Ferraris (among other trends from the 80s).
reducesuffering 5y ago
Is there data somewhere on optimal mph for fuel savings for the average sedan or by each car? I'm assuming it's basically the slowest you can maintain a decent rpm in the highest gear, but curious where that falls for different models.
- tagoregrtst 5y ago
  It depends (a lot) on your car and the engine. In general your otto-cycle (diesel is different) engine is most efficient at full throttle near its peak torque. However, this is far more power than is needed at cruise. So the lowest speed your highest gear can go? Not quite. First, engines are so powerful that you’re probably well in a range dominated by aerodynamic forces. With modern 10 speed transmissions you are probably several speeds too high. Like I said, it depends on the car, you’d have to test it on an empty flat country road. But I’d guess 50 kph or less. I’m a pretty staunch Republican, but Jimmy Carter was a great president. And his 55 mph interstate speed limit is the most sensical fuel economy policy of any president. Then again, Jimmy is an engineer :)
  - adrianN 5y ago
    Modern engines change the fuel air mixture if you go full throttle to give you extra power at the expense of efficiency. You really need to look at the engine spec to find the most efficient operations regime.

tenthirtyam 5y ago

Hmm. But over a fixed distance, the overall energy expenditure scales as v^2 (for v^3 power regime). Double the speed implies octuple (?) the power but only for half the time implies quadruple the energy spend. Not invalidating your point, but just a minor adjustment. :)

tagoregrtst 5y ago
I love it :) Never considered that point but, as a rebuttal, consider that trip time vs speed is extremely non linear and stochastic (due to traffic, lights, etc). In my experience you have drive much faster than x% to increase your average speed x% and it doesn't appear to be a linear scaling at all. Of course, you shouldn't be doing 80 in urban roads

nerdponx 5y ago

Subjectively it feels like highway speeds have been going up steadily over the last 10 years. Maybe not. But I don't like that "cruising" speed on US interstates is 70-75. Seems dangerous and wasteful, just to save a few minutes travel time. But then again people also drive on the highways with huge empty pickup trucks all the time, so they probably just don't care about waste.

danans 5y ago
> Subjectively it feels like highway speeds have been going up steadily over the last 10 years. Maybe not. But I don't like that "cruising" speed on US interstates is 70-75 Secretly, I hope that range anxiety in EV sticks around a little longer so that people lower their average speeds on the freeway to max out their range. That, coupled with a growing realization that most of the fun with driving is in the peppy acceleration and good handling which is pretty much table stakes for modern EVs across the price spectrum. High sustained top speed in a straight line with an overpowered ICE car is boring and should be buried with the Lamborghinis and Ferraris (among other trends from the 80s).

reducesuffering 5y ago

Is there data somewhere on optimal mph for fuel savings for the average sedan or by each car? I'm assuming it's basically the slowest you can maintain a decent rpm in the highest gear, but curious where that falls for different models.

tagoregrtst 5y ago
It depends (a lot) on your car and the engine. In general your otto-cycle (diesel is different) engine is most efficient at full throttle near its peak torque. However, this is far more power than is needed at cruise. So the lowest speed your highest gear can go? Not quite. First, engines are so powerful that you’re probably well in a range dominated by aerodynamic forces. With modern 10 speed transmissions you are probably several speeds too high. Like I said, it depends on the car, you’d have to test it on an empty flat country road. But I’d guess 50 kph or less. I’m a pretty staunch Republican, but Jimmy Carter was a great president. And his 55 mph interstate speed limit is the most sensical fuel economy policy of any president. Then again, Jimmy is an engineer :)
- adrianN 5y ago
  Modern engines change the fuel air mixture if you go full throttle to give you extra power at the expense of efficiency. You really need to look at the engine spec to find the most efficient operations regime.