Stay within posted speed limits. Gas mileage decreases rapidly at speeds above 60 miles per hour.
Avoid unnecessary idling. It wastes fuel, costs you money, and pollutes the air. Turn off the engine if you anticipate a wait.
Remove excess weight from the trunk. An extra 100 pounds in the trunk can reduce a typical car’s fuel economy by up to two percent.
Change your oil. According to the U.S. Department of Energy (DOE) and Environmental Protection Agency (EPA), you can improve your gas mileage by using the manufacturer’s recommended grade of motor oil. Motor oil that says “Energy Conserving” on the performance symbol of the American Petroleum Institute contains friction-reducing additives that can improve fuel economy.
Check and replace air filters regularly. Replacing clogged filters can increase gas mileage up to ten percent.
Your owner’s manual recommends the most effective octane level for your car. For most cars, the recommended gasoline is regular octane. In most cases, using a higher octane gas than the manufacturer recommends offers no benefit. Unless your engine is knocking, buying higher octane gasoline is a waste of money.
Hybrid Electric Vehicles offer another option for car buyers. According to DOE and EPA, these vehicles combine the benefits of gasoline engines and electric motors and can be configured to achieve different objectives, such as improved fuel economy and increased power.
Be skeptical of claims for devices that will “boost your mileage by an extra 6 miles per gallon,” “improve your fuel economy up to 26 percent,” or the like. EPA has tested over 100 supposed gas-saving devices — including mixture “enhancers” and fuel line magnets — and found that very few provide any fuel economy benefits. The devices that work provide only marginal improvements. Some “gas-saving” devices may damage a car’s engine or increase exhaust emissions. For more information and a full list of tested products, check this.
The site has a few more but you get the idea. Keep your car well maintained and drive the posted speed limit.