How Vehicles Are Tested
Fuel economy is measured under controlled conditions in a laboratory using a standardized test procedure 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 10%–15% of them through their own tests at the National Vehicles and Fuel Emissions Laboratory.
Estimating MPG with Laboratory Tests
In the laboratory, the vehicle's drive wheels are placed on a machine called a dynamometer that simulates the driving environment—much like an exercise bike simulates cycling.
The energy required to move the rollers can be adjusted to account for wind resistance and the vehicle's weight.
Measuring Fuel Use
For vehicles using carbon-based fuels (e.g., gasoline, diesel, natural gas, etc.), a hose is connected to the tailpipe to collect the engine exhaust during the tests.
The carbon in the exhaust is measured to calculate the amount of fuel burned during the test. This is more accurate than using a fuel gauge.
This method does not work for vehicles using non-carbon-based fuels, such as fuel cell vehicles and electric vehicles.