ya higher speeds lower gas mileage and wear the tread of your tires quicked
a bugatti veyron's tires last only 20 minutes going at its top speed
its all because of wind resistance
i forget the exact numbers but i think its like once you go past 60mph 80% of the energy is used to fight the wind (the numbers may be off ill look it up later)
hey is a smart car one of those little japanese bubbly things (if not then plz ignore how stewpid i look, its just that thats the only thing i can think of that gets a jillion mpg and is 2 seater)