[DoctorP82]

You are also going faster than you are when doing city driving at a stop-go rate. My vehicle can do 1500 rpm at both 25 and 55 mph, but i'm covering more ground per revolution at 55. Of course there are issues of drag, but if you look at most EPA estimates for cars fuel economy, universally the highway mileage is higher. I'm pretty certain that outside the fact you are able to travel constant rather than regular accel/decel, it is that you are in a higher gear at an RPM rate close to what you would have in a lower gear in slower speeds.