Fuel efficiency

That's zero gallons per hour, but also zero miles per gallon... Engine off in neutral on a smooth pavement and a heck of a wind, now maybe that'll get you some MPG, but your choice of direction might be a trifle limited.

Seriously, the longest range (implying highest MPG) speed for actually going somewhere would depend a lot on the car -- what the power curve of its engine looks like and how it's geared and what sort of pumping losses the engine has and how aerodynamic the car is.

In principle, I reckon that you would find a broad optimum around the lowest speed your engine can comfortably sustain in its highest gear. In terms of specific speeds, that used to be around 35-40 for typical cars, but these days it is often more like 50-55.

presents a curve showing this. Though presented without on-the-spot explanation, there is a reference to the technical document whence it came:

The most specific element of that interesting and valuable but BIG! document is table 4-24 on page 4-27, in

Though not as deadly important a subject for on-road motorists as it is for aviators and offshore powerboaters, the best-range speed for cars in general and yours in particular, under various circumstances, can be a good thing to know. Maybe the next town is further away than you thought, or the boondocks gas station you were counting on has locked up for the night, or you have to calculate whether it's better to press on or double back or just pull over at the next safe place and call the auto club...

Some cars these days have a trip computer that can show your instantaneous mileage and estimate the remaining range. Good feature, though you should also get a feel for how much truth it tells, since it might know the speed and maybe even the fuel flow rate pretty accurately but is probably getting the amount remaining in the tank from the same sensor as the guess gauge in the dashboard. (Some of those are accurate but others, including the unit in one of my cars, should really be marked, "Intended as an amusement device only -- not intended for gambling purposes.")

Anyway, if you need to stretch your last dollar to the next paycheck or your last inch of fuel to the next town, accelerate smoothly to 50-55 (buttuming a newish car) and steady on. Preferably not in the left lane, if you are doing this on the freeway.

Cheers, --Joe