Your engine is optimized for 70mph. At 140, you will notice increased fuel consumption due to drag. By the way, just what do you think limits your speed? It is simple: AIR RESISTANCE. Once you reach a certain point, your car will not be able to go faster. This is entirely an issue of air drag, mechanical friction, and vibration. The horsepower of the engine simply cannot overcome these sources of friction. The thrust of your engine exactly matches the combined affect of drag and friction.
Jets go faster because they have much higher horsepower ratings, all to overcome air drag.
Physics is just the way things work.
All that is wonderful, but my response was to your assertion that it was impossible for billbears' car to get better gas mileage at 70 than at 55. You just rebutted your own argument.
By the way, mechanics is just the way things work, physics is the rules. Mechanics allow me to have a car that gets better gas mileage at 70 or 90 than at 55.