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If you are running low on fuel in a gasoline powered car I guess driving at a constant speed is best and turning off stuff like the AC would be wise .

I suppose keeping the car in a reasonably high gear would be good too?

The question is what is the best speed?

If too fast air drag will suck the gas but if too slow the poor part load efficiency of the Internal combustion engine would burn the fuel anyway.

Is this speed higher for larger cars?

Is this speed quite broad ?

Is it different for a diesel ?

What would be the story for an electric car ?

would the optimum speed be real slow ?

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  • I would say at constant speed the most economical would be top gear, minimum revs and minimum throttle, as that way you cover the largest distance with minimum fuel injected. However for acceleration I've heard many opinions. Some say feather throttling to desired speed to consume less fuel, others say floor it to get to speed faster, the rest say ~75% throttle is the best for economy. Honestly, I would like to know as well. – I have no idea what I'm doing Feb 10 '16 at 10:11
  • Certainly for older cars in Europe, 55mph would be your target speed because MPG figures used to be quoted in the motoring press for a constant 55mph so manufacturers would tune their cars to be most efficient at that speed. – Steve Matthews Feb 10 '16 at 11:47
  • Although, "What speed in a car most economical?" the answer technically is stationary with the ignition switched off. You won't burn any of the remaining fuel in that state. – Steve Matthews Feb 10 '16 at 11:48
  • My friend's subaru legacy gets the best MPG at over 80 mph for extended periods. My Dodge Ram 1500 (auto) gets the best mpg at 40-45 mpg, steady. That's the lowest speed that the overdrive kicks in, so @Ihaveno is right at least about my truck – J. Musser Feb 10 '16 at 14:39
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    Related, answer to a question posted shortly before this one: mechanics.stackexchange.com/a/25890/12030 – JPhi1618 Feb 10 '16 at 17:33

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