My fishing buddy and I are having a heated debate over which has a greater effect on gas mileage, engine speed or wind resistance, assuming all other factors are optimum and constant. Assume two identical cars (say, Ford Expeditions) are being driven identically on identical roads, with one going 70 mph and the other going 60. The one going 70 will have a lower gas mileage obviously, but what percentage of the drop will be attributable to each factor, rpm and wind resistance. Please answer asap this so we can get back to fishing.

Wind resistance has a far bigger effect at higher road speeds. The resistance increases with the square of the speed.

Tom & Ray answered this same question a few years back in one of their newpaper columns. See:

http://www.thefreelibrary.com/CAR+TALK\Wind+resistance+key+to+gas+mileage+at+high+speeds.-a083901363

It’s wind resistance and, to a lesser degree, road resistance. To really answer your question, you’d want to see what the mpgs are at a constant speed, say 60 mph, when you change rpms by changing gears. You’ll probably get slightly worse mpgs in 4th than in 5th, for example. But the drop in mpgs between 60 mph and 70 mph is almost all due to wind and road resistance.

Joe is correct, but taking it a bit further, revolutions per minute has virtually no effect on gas mileage. It is revolutions per mile that matter. Using your two identical cars as an example, if one drops to a lower gear and they travel the same speed, the car in the lower gear will have higher RPM, but what matters is that that engine will turn more revolutions per mile, and will therefore fire more cylinders full of gasoline mixture per mile, and will consume more fuel per mile.

This effect is not nearly so pronounced with a diesel as it is with a gasoline engine, but I won’t muddy the waters by going in to that discussion.

At speeds of 60 and 70 mph the single biggest factor to mileage is wind resistance. Second is road resistance.

the next time you’re on the highway, put your hand out the window with the palm forward. You’ll need substantial push to keep your hand stable.

Your hand is about 6" by 4", or 24 square inches.

Let’s say that you’re vehicle “cross section” is 6’x4’ (96" x 48"), or 4,608 square inches. That’s 192 times the effort you’re expending.

Aerodynamics is a bit more complicated than my example, how efficiently the 4,608 square inches of air is moved is a critical determinant of how much effort it takes, but it takes a lot of effort to keep a car moving through the air at highway speeds.

Thanks…chemengr