The Internet has been buzzing with calculations, many of them erroneous, about how General Motors arrived at its estimate of 230 mpg equivalent for the series hybrid Chevy Volt, due late next year.

The smoke has cleared enough that we can get a pretty clear picture. First, the Environmental Protection Agency tells me that the draft methodology used to arrive at 230 mpg was sent out to all interested carmakers, not just GM. In fact, Nissan used it to estimate its forthcoming Leaf electric car at a stellar 367 mpg.

"The 367-mpg equivalency estimate is based on the Department of Energy's regulation that is used to calculate fuel economy values for zero-emission vehicles," says Nissan's Steve Oldham. "These calculations are primarily used to assign values for [Corporate Average Fuel Economy] CAFE standards." If you're good with numbers and legalese, the formula is online here.

Tony Posawatz, vehicle line director for the Volt, told me that its 230-mpg calculation was based both on the EPA/DOE preliminary data and its own real-world testing. Larry Nitz, GM's executive director of hybrid powertrain engineering, clarified in a conversation with GM-Volt.com (not affiliated with GM) that (as author Lyle Dennis summarized it) "the average Volt driver charging his car nightly can expect to burn one gallon of gas for every 230 miles traveled over time based on the behavior of a particular random population that was studied in 2001."

That does actually make sense. The Volt will only get incredible fuel economy if people don't drive a whole lot every day, and GM's research (from eight years ago) shows that they don't. Are we in for mpg wars, in which three-digit numbers will become routine on window stickers?

No t exactly, says Paul Weissler in an article for the Society of Automotive Engineers. Weissler points out that CAFE numbers are not the same as window sticker numbers. In fact, the MINI E battery electric car, which I drove earlier this week (lots of fun) has a window sticker of 102/94 equivalent mpg.

CAFE numbers are based on a complicated mathematical formula, Weissler says, but since 2008 the EPA requires dynometer testing based on five driving cycles for window stickers. Unfortunately, there is no five-cycle test for electric cars yet, so the EPA has some alternatives using correction factors of up to 30 percent. Most EVs will end with combined EPA sticker mileage of about 100 mpg, Weissler said.

The stickers could be somewhat misleading at first glance: The big black numbers (33/36) actually refer to kilowatt-hours of consumption in 100 miles. You have to read the fine print to get to the 102/94 mpg.

Flickr photo

View CBS News In