In general, the faster you go, the more gas you use. The defunct "Drive 55" campaign in the U.S.A. was not only a safety measure, it was also an energy conservation measure: cars use less gas at 55 mph (90 kph) than at 65 mph (105 kph). Leave ten minutes earlier than you planned to, and drive a little slower than you normally do. As a bonus, you'll have more reaction time and may keep yourself out of an accident.

• Heavy acceleration and hard braking greatly reduce gas mileage. Smooth acceleration from traffic lights and gradual braking at stop signs will help more than you might think. And if you're driving through a residential area, you'll make it a safer and more pleasant place for the folks who live there.
• Constant speeds, rather than speeding up and slowing down, help enormously. That's why highway gas mileage estimates are always higher than the estimates for around-town driving. If you can plan your route to avoid strings of traffic lights, stop signs or children's play areas, your gas mileage will definitely improve.
• In most cars, the air conditioner draws power from the engine, using a belt. Every time that you turn on the air conditioner, the engine has to use more gas to keep the car moving. Although opening the windows can also reduce your gas mileage by reducing the aerodynamic efficiency, it's still better than running the air conditioner.
• Overloading the car makes the engine work overly hard and consume extra gas. If you can split the luggage (or building supplies) between two vehicles, then do it. This doesn't mean, however, that you ought to take two cars when one car will suffice.