Next month, Mitsubishi will introduce a subcompact car with some hefty bragging rights. The 2012 Mitsubishi i stands as America's most fuel-efficient car, at 126 miles per gallon in city driving, according to the Environmental Protection Agency (EPA).

Such impressive fuel economy seemed impossible a few years ago. (The 2012 Toyota Prius gets "only" 52 m.p.g.) And, in fact, 126 m.p.g. will still be impossible when the car goes on sale in January.

You see, the Mitsubishi i is an all-electric vehicle, so there aren't really any gallons for it to gulp. A closer look at the EPA window sticker reveals that it's actually 126 miles per gallon equivalent (MPGe). The EPA rolled out this new term to help translate electric-car efficiency into a figure that most Americans understand.

It's a metric that's embraced by the auto industry, but it has some science geeks wrinkling their noses.

"We had a lot of our technical people that were excited about using kilowatts per hour or per mile, but when you go and talk to the general public, they are very familiar with m.p.g. and they love m.p.g," says Margo Oge, director of the EPA's Office of Transportation and Air Quality. "So we came up with MPGe."

MPGe works well as a shorthand, but it doesn't tell the whole story. Let's look at how the EPA calculates MPGe.

The agency needed some way to compare two very different power sources. Gasoline and electricity use completely different units (gallons versus kilowatt-hours) so you can't just shove them into the same formula. Then someone came up with a very clever solution: If you burned a gallon of gasoline, it would generate 115,000 British thermal units of heat. So, how much electricity would it take to generate the same amount of heat? Answer: 34 kilowatt-hours (kWh). We now have our connection. One gallon of gasoline produces the same amount of energy as 34 kWh.

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With this new comparison point, the EPA can now figure out how far an electric car will travel on only 34 kWh. In the case of the Mitsubishi i, that much electricity will carry it for 126 miles. So, it's 126 miles per the equivalent of a gallon of gas. Voila! You have MPGe.

But when car shopping, holding up MPGe to m.p.g. isn't really a fair fight. It compares energy consumption, not fuel costs. Just because one gallon of gas equals 34 kWh in terms of energy does not mean that they cost the same amount of money. (See chart above.)

Wallet-conscious car shoppers should instead focus on a different figure: kilowatt-hours per 100 miles. When 2013 vehicles arrive this spring, they'll be required to list this additional statistic, but in much smaller print. Seek it out. It makes actual pocketbook calculations much easier.

For example, take the 2012 Chevrolet Volt, which offers all-electric driving with a gasoline tank as backup. Its EPA label touts both 37 m.p.g. and 93 MPGe. Those are strong numbers for either mode, but how much cheaper is one mode than the other? Check the fine print. It says that the Volt takes 36 kWh of electricity to drive 100 miles. Or, when you switch to gasoline, it's 2.7 gallons per 100 miles. Now it's just a simple calculation.

Multiply 36 kWh by however much you pay per kWh on your home electri­city bill. Let's say it's 12 cents per kWh, the national average. So, 36 kWh times 12 cents gets you your cost: $4.32 to drive 100 miles using only electricity.

Then, multiply the EPA's gallon figure by the price at your local gas pump: 2.7 gallons multiplied by the national average of $3.275 per gallon, which equals $8.84 to drive 100 miles on gasoline.

Now you're comparing apples to apples. You can pay $4.32 by sticking to the battery, or more than twice that to drive that same distance using the less efficient gasoline mode.

While dealers are not required to post such numbers on 2012 models, you can find all this information at www.fueleconomy.gov. The website also offers many more helpful stats, such as how far each electric car will travel on a single charge, how long it takes to fill up the battery, and how much the average driver will save by choosing a more fuel-efficient vehicle.

For more on how technology intersects daily life, follow Chris on Twitter @venturenaut.