Your engine only makes the power needed at the time, eg.. say 65 hp at 70 mph, it’s gonna take a certain amount of fuel within a narrow range to do so, how efficient your engine is making 65 hp at cruising speed is what matters.
""At 60 mph, a typical car needs 10 to 20 horsepower simply to maintain its speed. That energy level is needed to overcome wind resistance and the rolling resistance in the tires.""
