The US Energy Department says aggressive driving (including rapid acceleration and hard braking as well as speeding) wastes gas. It can lower your highway gas mileage 33 percent and city mileage 5 percent.
Using cruise control is one way to smooth out highway rides. Scheduling your work hours to avoid peak rush hours can help you commute both more quickly and with better gas mileage.
Another pointer: Reduce aerodynamic drag by putting stuff inside the car rather than on roof racks when possible. Driving down the highway with your windows open also creates drag.