• EVs don't burn gas but still get a fuel economy rating
  • MPGe stands for miles per gallon-equivalent
  • kWh/100 mi stands for kilowatt hours of electricity used per 100 miles

Gasoline prices are skyrocketing again, and with more plug-in choices showing up every month, it's a new world for car buyers, many of whom may be thinking of taking the plug-in plunge. But with that new world come brand-new EPA window stickers to look at and decipher on the dealership lot.

The window stickers on alternatively fueled vehicles include a number of new distinctions. The most basic is what type of car it is: all-electric or plug-in. As their name implies, all-electric vehicles use only electricity to move the vehicle forward. They depend on large batteries to provide all that energy. Plug-in hybrids have smaller batteries and can travel a much shorter distance while powered by electricity from a wall outlet. They also have a combustion engine on board that burns gasoline or diesel to extend the car's range between charges.

The measure of miles per gallon has been used in the United States for many decades to describe how fuel-efficient a car is. When running on electricity from a wall outlet, neither type of plug-in car burns any fuel, but even so the EPA wanted to provide consumers with a familiar way to compare these vehicles' efficiency with that of a conventional gas- or diesel-burning car.

As a result, plug-in cars running on electricity also have an MPG rating from the EPA, which is a bit counterintuitive since plug-ins using electricity don't burn gallons of anything. Even so, when they are drawing electricity from the battery, plug-ins are using an equivalent amount of energy to move forward, so the fuel economy measure for them is called miles per gallon-equivalent, or MPGe. MPGe is based on the EPA's calculation that every gallon of gasoline contains the equivalent amount of energy contained in 33.7 kilowatt hours (kWh) of electricity (kilowatt hours being the way electricity is measured).

MPGe can be useful as a gauge for comparing how efficient a plug-in car is when running on electricity. Generally, that's two to four times more efficient than a comparable internal-combustion-powered vehicle. However, MPGe doesn't really help decipher how much that car will cost to drive using electricity alone.

For consumers who want to calculate exactly how much their electric utility bill might go up, the new window sticker also tells you how many kilowatt hours a given plug-in car will use in 100 miles (kWh/100 mi) when driving on electricity from a wall outlet. For kWh/100mi, the smaller number is the more efficient number - the opposite of a traditional MPG figure. For comparison, the Nissan Leaf is rated at 34 kWh per 100 miles, the Mitsubishi i at 30 kWh/100 mi, the Ford Focus Electric at 32 kWh/100 mi, and, when running on electricity from a wall outlet, the Chevrolet Volt is rated at 36 kWh/100 mi.

To estimate the cost per mile to drive any plug-in on electricity, simply find the charge per kilowatt hour on your current utility bill. The average for the U.S. is about 12 cents. Multiply it by the car's kWh/100 mi rating, and then divide by 100. For example, using the U.S. average, the Nissan Leaf will cost about four cents per mile to drive.

The new window stickers also tell you how far each plug-in vehicle can travel on one charge of electricity from a wall outlet. In the case of all-electric vehicles, that's the number of miles you'll be able to go before needing to recharge. For plug-in hybrids, it's the number of miles you can drive before the range-extending combustion engine turns on.

The range and efficiency numbers listed on the new window stickers can vary by as much as 25 percent based on driving habits, average speeds and external temperatures.

What it means to you: If gas prices have you scrambling and you're thinking about buying a plug-in car, arm yourself with an understanding of how to read and use the new window stickers before you take the plunge.

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