While the overall force required to push a vehicle through the air is definitely higher as speed increases, that doesn't necessarily mean that fuel efficiency decreases over a given speed range increase.
Miles per gallon used at a given speed depends on several factors, including the engine displacement, the RPMs at which the engine needs to turn to maintain that speed - and that is dependent on the overall drive ratio, the weight of the vehicle (including its load), the efficiency of the drivetrain, and the rolling resistance of the tires.
Sure, if you had two identical vehicles with identical drivetrains each tuned for maximum efficiency at a certain speed without considering any other factors, the one tuned for 55 will get better mileage than the one tuned for 75.
However, its very possible that a given vehicle will give better mileage at the higher of two speeds, depending on how that mix of factors combines for that particular vehicle at those two speeds.
For a normal vehicle that might be true if the difference in the speeds was say 60 vs. 55 but for 55 and 75 the relative velocities for a given Cd preclude that.