• Vehicle maintenance: A poorly tuned engine burns more fuel, so fuel economy will suffer if your engine is not in tune. Keeping tires at the correct pressure and changing the air filter on a regular basis can improve fuel economy. Also, new energy-saving motor oils can improve MPG.
• Aggressive driving (speeding and rapid acceleration and braking) can decrease fuel economy by as much as 33 percent at highway speeds and 5 percent around town. This can cost you as much as $0.49 per gallon.
• Driving at high speeds creates additional wind resistance, which reduces fuel economy. Each 5 miles per hour you drive over 60 mph is like paying an additional $0.10 per gallon.
• Excessive idling will decrease your average mpg, since idling gets 0 miles per gallon.
• Towing a trailer or carrying excessive weight can decrease fuel economy.
• Running electric accessories such as your air conditioner can decrease fuel economy. Limited, informal tests by EPA have shown a fuel economy reduction of roughly 5 to 25 percent for operating the air conditioner on "Max" compared to not using the air conditioner.
• Frequent short trips can reduce fuel economy. Your engine doesn't operate efficiently until it is warmed up. In colder weather, it takes longer for your engine to warm, and on short trips, your vehicle operates a smaller percentage of time at the desired temperature.
• Driving on hilly or mountainous terrain or on unpaved roads can reduce fuel economy.
• Using 4-wheel drive will reduce fuel economy. Four-wheel drive vehicles are tested in 2-wheel drive. Engaging all four wheels makes the engine work harder and increases crankcase losses.
• Fuels vary in energy content: A vehicle's fuel economy depends on the energy content of the fuel on which it runs-this has been determined by EPA and others and is consistent with combustion theory. The use of oxygenated fuels or reformulated gasoline (RFG), for example, can cause a small decrease (1-3%) in fuel economy. In addition, the energy content of gasoline varies from season to season. Typical summer conventional gasoline contains about 1.7% more energy than typical winter conventional gasoline.

From www.fuelecononmy.gov