fb tracking

What’s my MPG? Plug-ins are in limbo

Today General Motors made yet another big, splashy announcement about the Chevy Volt – complete with a hip-to-the-youngsters viral marketing campaign. This time the news is that GM estimates that the Volt will get 230 miles per gallon in city driving.

So…what does that mean? For the plug-in hybrid Volt, it’s anyone’s guess. The Environmental Protection Agency (EPA) calculates fuel economy ratings for electric vehicles by measuring the efficiency of the vehicle running on electricity and converting to miles per gallon. (For more details on this see this piece by the Society of Automotive Engineers.) For hybrids like the Prius, EPA tests vehicles using a similar test to what they use for conventional vehicles. But plug-in hybrids, which straddle the line between electric vehicles and hybrid vehicles, in a sense, could go either way.

The Volt is an all-electric vehicle for about 40 miles, according to GM. In this range, an estimate based on EPA’s electric vehicle calculation would make sense. But, as hybrid manufacturers and owners found in 2008, short-range, city driving estimates don’t tell the whole story.

Edmunds.com explained in 2008:

In general, in order to get the highest window sticker possible, ultrahigh fuel economy vehicles like hybrids have, more than any other class of vehicle, been developed and tuned around the idea of doing well on the FTP and HFET test cycles. Reports from consumers that hybrids don’t get anything close to the label values are an illustration of how unrealistic the outgoing tests have been, and illustrate the need for a realistic test method since manufacturers will “teach to the test” as much as they can.

This difference is even more pronounced with a plug-in hybrid vehicle, where the distinction between all-electric operation and gas-electric hybrid operation is more pronounced. It’s not an easy question to answer, but EPA must make an effort before these vehicles go on sale, and consumers want to know how to compare the Volt with other vehicles.