In school we learned ” what you save in energy, you have to increase the way.” By that rule you should use the exact same amount of energy (fuel) for the same distance no matter what speed. I’ve asked a few people, but no-one could give me a good answer.
In: 15
There are 3 major sources of fuel consumption
Baseline – this is pretty constant and is the fuel needed just to keep the engine spinning at any speed
Rolling resistance – this is from the tires squishing. It scales linearly with speed so 40 mph requires twice as much power to overcome this as 20 mph but you cover twice as much distance so it washes out
Air resistance – this scales with the square of speed so 80 mph takes 4x more power to over come air resistance than 40 mph
The end result is that a plot of speed vs fuel use per mile looks like a bowl with a low spot somewhere in the center and higher consumption at really low and really high speeds.
At low speeds, the fuel required to keep your engine spinning dominates and you aren’t covering much distance so the result is pretty terrible.
At high speeds air resistance dominates all else and going from 100 mph to 110 mph will increase fuel consumption by at least 21% despite only increasing speed 10%.
The range in the middle is where it gets tricky and depends on the car. Modern cars have lower rolling resistance tires so item 2 above is minimized, they’re also far more aerodynamic which reduces air resistance and changes the slope of the curve. They’re generally pretty good in the 40-75 mph range these days, partly thanks to the increase in gears(3 speed autos have been replaced by 9 speeds) so manufacturers don’t have to pick just a single upper speed that is in the engine’s happy range.
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