Source: http://www.knightopia.com/journal/archives/000779.html
Find the places near you that offer the cheapest gas and shop there first. (Resources: Cheap Gas, Gas Buddy, Gas Price Watch)
Check your tire pressure. Under-inflated tires require more energy to roll. The placard on the inside drivers door is where you'll find the correct tire pressure (usually about 28 PSI). Consider replacing worn tires with Low Rolling Resistance (LRR) tires.
Fill up on weekdays—because prices usually rise on the weekend. The cheapest times to buy gas are typically Tuesday midday or Wednesday morning.
Fill up at night because pumps deliver more gas when temperatures are lower.
Buy gas from busy gas stations. Stations that are consistently busy have their tanks refilled regularly, which means the fuel is more likely to be fresh. Fresh gas has more power than gas that has become contaminated by sitting in infrequently used tanks.
Avoid gas stations near freeways, however, because prices are often higher there.
Don't overbuy. Unless your owner's manual says you must use a higher grade (which very few cars actually do), buy regular gasoline. Costlier high-octane gas does not improve the performance of your vehicle.
Avoid topping off. When you purchase just a bit of gas at the gas station the pump doesn’t have enough time to really activate, resulting in short bursts of fuel that may short change you from the amount of gas that you are purchasing.
Avoid running on empty. You may think you're using very little gas when your car is on empty, but you're actually using more gas because your vehicle is running less efficiently as it tries to accelerate and decelerate in a normal fashion. The best time to replenish your gas tank is when you have half a tank or less left, or when you find a gas price that you just can’t pass up.
Carpool whenever possible. Carpoolers often share the cost of gas, and the wear'n'tear on your vehicle is reduced as well.
Combine trips. Several short trips taken from a cold start can use twice as much fuel as one trip covering the same distance when the engine is warm.
Choose your vehicle wisely. If you own more than one vehicle, drive the one that gets better mileage whenever you have the option.
Choose your route wisely. Take the route with the flattest terrain and fewest stops. Try to avoid traffic jams and stop and go traffic.
Slow down. Driving at posted speed limits saves fuel and lives. Each 5 mph over 60 mph is like paying an additional 10 cents per gallon. Use cruise control to maintain your speed.
Avoid sudden stops and starts. Erratic acceleration and braking can waste up to 50 cents a gallon.
Avoid idling. If you have to sit in your car for more than one minute, shut off the engine.
Limit air conditioning, which consumes fuel. At lower speeds, open the windows or sunroof to stay cool. At higher speeds, use the car’s fan instead (open windows create drag that reduces mileage).
Park in the shade. This minimizes fuel evaporation and helps keep your car cool in the summer.
Lighten your load. Don’t carry around items you don’t need. For every 100 pounds of weight in your car, fuel economy decreases by one to two percent. Put heavy items in the trunk instead of on a roof rack, which creates drag.
Keep your gas cap on tight. Tightening the gas cap on your car will prevent gas from evaporating and escaping into the air. If you've lost your gas cap, buy a new one ASAP.
Change your oil, spark plugs, and air filter on schedule. And go to a repair shop if your “Check Engine” light comes on—a faulty oxygen sensor could be the cause, lowering your mileage significantly.
Use the recommended grade of motor oil, preferably one with "energy conserving" on the label. Gas mileage could improve 1-2 percent.
Get a tune-up. A simple tune-up on your car can improve your gas mileage by an average of 4.1 percent.
Look into discount gas card offers. Shell, Gulf, BP-Amoco, Exxon-Mobile, and others have rebate programs that offer as much as 5-10 percent off the gas you buy.
Track your average mileage and have your car checked if the average decreases. To calculate your mileage, note the odometer reading and number of gallons purchased each time you fill up. Divide the number of miles traveled between fill-ups by the number of gallons purchased.
