How we gauged fuel economy for the Chevrolet Volt

Consumer Reports News: August 24, 2011 09:23 AM

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Assigning an appropriate fuel-economy score to the Chevrolet Volt posed special challenges. Fuel use depends on how frequently the car is charged and is determined by how much the car is run on electricity or gas.

We put the Volt through the same city, highway, and mixed-road trip cycles we use for other cars. We doubled the cycles to account for all-electric and gas modes and used an onboard measuring device to track the energy the car used for a given distance. Comparing those findings to what our charger put in showed us that the battery usually uses only 84 percent of the power that's input; the rest is lost to inefficiencies.

We converted our actual consumption in kilowatt-hours to a miles-per-gallon equivalent using a formula developed by the Environmental Protection Agency, which is currently the accepted standard.

To determine a typical-use scenario we assumed that a Volt owner would travel 50 miles a day, with 35 of those miles on electric power. We therefore weighted the electric portion at 70 percent. The EPA figures it at 65 percent, and General Motors reckons actual Volt owners rely on electric power for 66 percent of their driving.

See our full report from the October issue, and read the Volt road test and Full Track Report.