Road friction is fixed, regardless of speed, AFAIK. Drag through bearings is also fixed. Wind drag goes up as the square of speed. As pointed out, it might be possible to gear it so that the engine is at its sweet spot at 70. [I think it was on here that I read GM got into trouble for leaning out the ECU at 70mph? Below that speed, it ran richer to meet EPA requirements. May have been the typical internet controversy, though--I've never seen data for that assertion.]
I'd think that would make it look bad on the EPA test cycle though, since that means it's always "not a peak", since EPA doesn't get to 70mph AFAIK.