When driving on the highway, I drive slower... essentially, I drive the speed limit or maybe just below it.
This has three benefits: it reduces my carbon footprint because I use less gas, I save money, and I gain a slight amount of safety.
Much of the energy expended by a car at high speed is pushing through wind resistance. Wind resistance increases with speed. According to the
Dept. of Energy, driving 65 instead of 55 reduces MPG by 16%. Driving 75 reduces it
another 16%.
It's true driving slower means that I arrive a little later. But I consider it a worthwhile trade-off.