A new ad campaign aims to maximize the impact of the fuel economy ratings estimated by General Motors for the Chevrolet Volt: 230 mpg city and “over 100 mpg” overall. GM says the Volt, which it calls a “range extended electric vehicle,” achieved those numbers using a new draft test protocol for testing the fuel economy of plug-in hybrid vehicles such as the Volt.

The Environmental Protection Agency (EPA) says it has published no such protocol, even in draft form. EPA spokeswoman Cathy Milbourn says the agency is still working with the California Air Resources Board, the Department of Energy, and the Society of Automotive Engineers to develop the protocol.

At issue is how to account for the electricity used to power plug-in hybrid vehicles. Using the standard EPA fuel economy test protocol for gasoline cars, plug-in hybrid’s batteries would have to be kept fully charged during the test, meaning the cars would have to complete the test running only on gasoline. But this misses the purpose of a plug-in vehicle, which is to use electricity instead of gasoline for short trips within the range of the battery’s capacity. In other words, you have to allow the batteries to be discharged. But you can’t give electric cars a free ride, either. That electricity comes from somewhere and needs to be counted in what the car consumes.

When we tested a Toyota Prius converted to a plug-in hybrid vehicle, the fuel economy rose from 42 mpg for the standard hybrid to 67 mpg once we plugged it in. But including the electricity the car used, we only got the equivalent of 53 mpg. The batteries in our plug-in Prius would last about 35 miles before their charge ran out. After that, the car’s mileage dropped from the original 42 mpg to 40 mpg overall, because of the extra battery weight.

So the mileage varies more than most gas cars depending on where and how you drive it. So be cautious of these figures. While we expect electric and plug-in cars to be much more efficient than gasoline-powered cars, their fuel economy (and more importantly energy consumption) depends entirely on how and where they are driven.

Gas cars’ fuel economy also depends on whether they are driven more on the highway or in the city. For plug-in hybrid, the relationship is reversed. Gasoline-only fuel economy can approach infinity if the car is driven only on short trips and frequently recharged. Or it can drop close to the level of conventional cars (or theoretically even below) on the highway. In the end, it may be true that the Volt can get 230 mpg on an unknown, future EPA city cycle configured specifically for electrified cars. But today’s miles per gallon measurements do a poor job of capturing the real efficiency of tomorrow’s plug-in hybrid cars. Some other form of comparison, such as kilowatt hours per 100 miles, may provide a better comparison of efficiency going forward. (Learn how we test fuel economy on today's cars.)

As the old adage goes, “results may vary,” and this will be especially true with this new breed of automobile.

See our earlier report: 2011 Chevrolet Volt: Fuel economy estimated at 230 mpg city and see our Chevrolet Volt preview for more information.

See a related report at AutoBlog: "EPA backs away from GM claim of 230 mpg for Volt."

—Eric Evarts

Learn about driving green in the Consumer Reports special fuel economy section