Determining mpg for hybrid and electric vehicles

Have you ever looked at fuel-economy estimates on the sticker of an electric vehicle or hybrid? Just how does the Environmental Protection Agency (EPA) come up with a "miles per gallon" estimate for a car that doesn’t always use gallons of anything?

To make the integration of alternative fuels as simple as possible, the EPA developed a formula for determining the energy consumption of electric vehicles. In this equation, 33.7 kilowatt-hours of electricity is used as the equivalent of one gallon of gasoline and is described as mpg-e instead of the conventional mpg.

According to www.fueleconomy.gov, the EPA’s process for estimating the mileage of hybrid vehicles that use both gasoline and electric/battery power is a little more complicated. It involves starting with the battery pack at full charge and then running the car until the battery power is depleted. Once depleted, the amount of energy used is calculated by measuring how many kilowatt-hours of electricity were used to fully charge the battery and then factoring in how many miles it took to use up that energy.

With the battery decharged, EPA testing continues, running the vehicle on gasoline alone to measure fuel consumption. When the gasoline mileage is determined, the mpg-e number is added to the mpg figure to come up with a composite value that is used as the EPA energy-consumption estimate you’ll see on the window sticker in the dealership.

Note, too, that the EPA uses a dynamometer (basically a giant treadmill for cars) to run vehicles through this testing so, as always, your results may vary.