"No because drag, windresistance, whatever you want to call it increases as a square relative to speed"
That's the rub. And because of it, eventually all vehicles get worse mileage as they go faster (once the overcome the overhead of keeping AC, Alternator, PS pump.. etc running. This eventually comes pretty early on. That's why there's always talk of the national 55mph speed limit.
Or, to put it more simply, the reason you're travelling faster is because you pushed the gas pedal down further, right? You're not taking that into your "I got there faster = used less gas" equation.