Is comparing MPGe to MPG misleading?

Scott Benjamin

The new fuel economy labels provide more fuel efficiency information, including estimated annual fuel costs, savings and information on the vehicle's environmental impact.
The new fuel economy labels provide more fuel efficiency information, including estimated annual fuel costs, savings and information on the vehicle's environmental impact.
Alex Wong/Getty Images

For about the past 30 years or so, fuel economy labels created by the United States Environmental Protection Agency (EPA) have remained basically unchanged. You know the label we're talking about here, right? The oversized stickers that are plastered to the window of every new car and truck on the dealer lot. Those are actually called Monroney stickers, named for Senator Almer Stillwell "Mike" Monroney, a Democrat from Oklahoma who pushed for the Automobile Information Disclosure Act of 1958.

Anyway, those familiar window stickers are currently undergoing a major facelift, and some of the information found on the new labels might not make a whole lot of sense to you at first glance. Especially if you're looking at an all-electric or a plug-in hybrid electric vehicle. These types of vehicles now include an all-new fuel economy measurement: MPGe (which stands for miles per gallon equivalent). According to the EPA Web site, you should "think of this as being similar to MPG, but instead of presenting miles per gallon of the vehicle's fuel type, it represents the number of miles the vehicle can go using a quantity of fuel with the same energy content as a gallon of gasoline."

Sounds like the math might be a little complicated, doesn't it? Well, it's not so difficult when you break it all down into a simple formula:

Total miles driven x energy in one gallon of gasoline ÷ total energy of all fuels consumed = MPGe

So, using the all-electric Nissan Leaf as our example, and filling in the formula using the EPA's numbers (they tell us that one gallon of gasoline contains the same amount of energy as 33.7 kWh of electricity, by the way), it looks something like this:

100 miles driven x 33.7 kWh ÷ 34.0 kWh = 99 MPGe

The same formula is used for other alternative fuel vehicles, too, like those that run on compressed natural gas or hydrogen...but you'll have to do the math to convert the specific fuel's energy content to the equivalent of one gallon of gasoline. Oh, and you'll also have to know the total energy consumed. Ugh...this IS getting a little more complicated, I suppose. But it's accurate, and that's what counts.

So is comparing MPGe to MPG misleading? Not at all. A little bit confusing at first, but not misleading. It's a direct comparison -- that is, as long as you have all the numbers available to you (and if you're capable of doing the math).

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