Environmental Protection Agency Plug-in Vehicle Efficiency Ratings
EPA Ratings are likely more accurate than owner estimates
Battery Electric and Plug-in Hybrid range and mpge will often vary from EPA tests, especially on the open road. While it is expected that both range and mpge will drop outside a controlled environment, many BEVs and PHEVs surpass EPA estimates, unlike estimates for gasoline, hybrid, and diesel vehicles.
Be aware that it is very difficult to measure true electric range on PHEVs, since many versions start the gasoline engine often for optimum efficiency (masking true electric only range). If the PHEV in which you are interested actually runs in true BEV mode (without the gasoline engine starting during start up, accelerating, or high-speed driving), true electric range will be less than what you perceive.
EPA Mileage Testing
Fuel economy is measured under controlled conditions in a laboratory using a series of tests specified by federal law. Manufacturers test their own vehicles, usually pre-production prototypes, and report the results to EPA. EPA reviews the results and confirms about 15%–20% of them through their own tests at the National Vehicles and Fuel Emissions Laboratory.
Your Mileage Will Vary
How & where you drive
Vehicle condition & maintenance
Environment (high and low temperature ranges)
Therefore, the EPA ratings are a useful tool for comparing the fuel economies of different vehicles but may not accurately predict the average MPGe you will get.