Drag is an exponential force. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Since power is the rate of doing work, exerting four times the force at twice the speed requires eight times the power. So, until they design an engine that uses less fuel while generating 40hp than it uses while generating 100hp (a physical impossibility) your statement is going to be absolutely false.
All very interesting information but my truck gets some of its best gas mileage at 70 MPH. Why do they do wind tunnel tests if drag is unbeatable?
Not exponential, though, it's a simple speed-squared (drag) or speed-cubed (power required) relationship. So doubling speed will result in 4 times the drag, and 8 times the power required to go that speed. Going 75 mph rather than 60 results in the aerodynamic drag being (75/60)^2 = 56% higher, and 93% more power required to overcome the aerodynamic drag.