I think it has to do with air resistance mostly. Air resistance is the predominant form of resistance at higher speeds. I would guess a car with a good coefficient of drag <.33 for example maybe 50 mph is the best speed. A car with worse drag maybe 40 is the best speed. Assuming level ground of course. I get really good mileage at 80 mph down hills but high speeding tickets...

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra