A blanket statement like "the car will burn more fuel to traverse the same distance at 78mph as it will at 60mph" is silly. For some cars, that may indeed be true. But, those cars would need to have an otto cycle engine that runs at about 60% of the (RPM that produces maximum horsepower) at a ground speed of exactly 60MPH IN TOP GEAR. Thus, pushing the car to 78MPH in top gear (for example) would push that particular otto cycle engine significantly beyond it's most fuel-efficient operating RPM. I don't know of any cars that are designed that way, but I concede the possibility exists that there may be a car like that. But you wouldn't want to own it, as it would accelerate like a toddler pulling a little red wagon. :)
Many cars are most fuel-efficient at speeds above 70MPH. To make a car more fuel-efficient at lower speeds, gearing would have to be changed to reduce the car's top speed. For example, if you had a non-speed-limited car that had a maximum ground speed of 75MPH in top gear, that particular car would probably be very fuel-efficient at (for example) 50MPH in top gear. Of course, the 0-60 time of that beast would be something like, punch the accelerator, take a nap, wake up, remember to click "start" on your stopwatch, go back to sleep . . . (!) -Dave
