Drive in the highest gear

When you drive at a constant speed, try to use the highest gear possible. On a flat road you can keep a steady speed at an engine RPM that is only slightly higher than the stationary RPM. The engine revolves less when you are in a higher gear. Each revolution means fuel is injected, so the less the RPM, the less the fuel consumption. Beware: in mountainous regions, especially with a heavy load, you can overstrain the engine if you stay for too long to the highest gear possible.