Driving habits, the length of the trip, the weight of the vehicle and the terrain over which the vehicle is driven are the main factors affecting fuel economy. Accessories used in the vehicle, the maintenance level of the vehicle and excessive idling can also decrease fuel economy.
Driving habits are the most likely factor contributing to a decrease in fuel economy. Aggressive acceleration and heavy braking can decrease fuel economy by 33 percent while driving on the highway, according to the U.S. Department of Energy. Driving at high speeds can also reduce fuel economy, although the Environmental Protection Agency accounts for speeds up to 80 miles per hour when conducting the fuel economy test on a vehicle. Driving habits can also reduce city fuel economy to a lesser degree, since the EPA accounts for starts and stops and speed limits are lower.
Traveling short distances when the engine does not achieve its normal operating temperature, carrying or towing excessive weight and driving over hilly terrain can all reduce fuel efficiency. When the EPA tests a vehicle to calculate its fuel efficiency, it assumes the engine operates at its normal temperature, that the vehicle is carrying 300 pounds of passengers or cargo and that it is driving over flat terrain.