Driver Behavior & Driving Conditions
Quick acceleration and heavy braking reduces fuel economy by as much as 33 % at highway speeds and 5 % around town. EPA tests do not account for this kind of vigorous driving.
Excessive idling decreases average mpg. The EPA city test includes idling, but drivers that experience more idling can experience lower MPG.
Driving at higher speeds increases aerodynamic drag (wind resistance) and reduces fuel economy. The EPA test accounts for aerodynamic drag up to highway speeds of 60 mph, but drivers often exceed this speed.
Cold weather and frequent short trips can reduce fuel economy, since 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. Note: Letting your car idle to warm-up doesn't help your fuel economy, it actually uses more fuel and creates more pollution.
Cargo or cargo racks on top of your vehicle (e.g., cargo boxes, canoes, etc.) can increase aerodynamic drag and lower fuel economy. Vehicles are not tested with additional cargo on the exterior.
Towing a trailer or carrying excessive weight can decrease fuel economy. Vehicles are assumed to carry three hundred pounds of passengers and cargo.
Running electrical accessories (e.g., air conditioner) decreases fuel economy. Operating the air conditioner on "Max" can reduce MPG by roughly 5 - 25% compared to not using it.
Driving on hilly or mountainous terrain or on unpaved roads can reduce fuel economy. The EPA test assumes vehicles operate on flat ground.
Using 4-wheel drive reduces 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.
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A poorly tuned engine burns more fuel, so fuel economy will suffer if it is not in tune. Improperly aligned or inflated tires can lower fuel economy, as can a dirty air filter or brake drag.
Fuels Vary in Energy Content
Some fuels contain less energy than others. Using 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.
Inherent Variations in Vehicles
Small variations in the way vehicles are manufactured and assembled can cause MPG variations among vehicles of the same make and model. Usually, differences are small, but a few drivers will see a marked deviation from the EPA estimates.
New vehicles will not obtain their optimal fuel economy until the engine has broken in. This may take 3 - 5 thousand miles.