Yes. Warm air is less dense than cold air. That is why hot air balloons rise and airplanes need longer runway distance in hotter weather. Air resistance is a dominating factor in energy loss at highway speeds. I get about 10% better mileage when it's 80 vs. when it's 60.

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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