[Travis T. OJustice]

not, not really....! as gas engines work less efficiently at higher speeds....

Not necessarily. It depends on the gearing of the vehicle. Many vehicles are more efficient at 70 than 55, in spite of the increase in drag from air resistance.

[Secret Agent Man]

It could very well be that way, but the drag could very well cancel out any fuel savings as the engine has to work harder (even though more efficiently) because of increased wind resistance. 15 miles makes a big difference when you’re talking 55 and 70 mph.