How MPGe Works

2011 Chevrolet Volt Fuel Economy Label. Want to learn more? Check out these alternative fuel pictures!

The Chevy Volt from General Motors runs so efficiently on its electric motor that many owners report going more than 1,000 miles (1,609 kilometers) before having to refill the modest tank of its reserve gasoline engine. Talk show host and avid automobile collector Jay Leno reportedly went more than 11,000 miles (17,703 kilometers) in his Volt without refueling the gas tank [source: Garrett].

Life as a hybrid or full electric vehicle owner certainly has its perks: When everyone else is complaining about high fuel prices, you get to brag about how fuel costs are smaller than a rounding error in your monthly budget.

Ay, but here's the rub: Even though you might not pay as much for fuel as you would owning a straight-up, internal combustion-engined vehicle, electricity still isn't free. OK, so maybe it is if you've bolted a self-sufficient solar array to your roof, but work with us a moment. For most folks with electrically powered vehicles, the piper to be paid will not be the corner gas station, but the local electric utility. You may or may not come out ahead on fuel costs when all is said and done after buying an electric vehicle.

So if you're trying to decide between a battery-powered electric or hybrid car and a conventional, internal combustion-powered ride, how can you tell which one is the most efficient?

The answer lies in miles per gallon equivalent, or MPGe. As the name implies, it provides a handy way to compare the fuel efficiency of vehicles that run on different types of fuel sources. Obviously, you'd be hard-pressed to measure electricity in gallons, which is a measure of volume -- those twitchy little electrons just won't stay put in a container. So to give consumers a meaningful point of comparison in a marketplace bustling with choices, MPGe was born.

This article will answer some of the bigger questions about MPGe. Who thought of it? What are the numbers and formulas for figuring it out? What does it mean when MPGe appears on a window sticker or a television commercial for a particular car?

So put on your thinking cap, because up next, we'll look at how to crunch the numbers to figure out MPGe. It's actually not that complicated -- we promise!