Because EPA fuel-economy estimates are derived in an ideal laboratory setting, they are just estimates that will not reflect real-world driving conditions. There are many factors that will cause your actual mileage to vary with EPA estimates. Here are some examples:

- Accelerating quickly and braking excessively will reduce MPG by as much as 33% in highway driving and as much as 5% in city driving.
- Driving at highway speeds above 60 mph.
- Driving on hilly or mountainous terrain and unpaved roads.
- Taking too many short trips that require the gasoline engine to run more than the electric motor in order to warm the emissions system.
- Carrying extra weight or towing a trailer. (The EPA test assumes only 300 lbs of passengers and cargo.)
- Using cargo racks. (Vehicles are tested without cargo racks, which can increase wind drag.)
- Poorly maintained vehicle. (Vehicles tested are in top condition.)
- Driving a brand-new vehicle. (Optimum fuel economy may not be realized until the engine is “broken-in,” which may take up to 5,000 miles.)

Beginning with the 2008 model year, the EPA mileage estimates will utilize different calculation methods that will take into consideration some of these factors, which should result in more realistic figures.