How MPGe Works

Comparing Automotive Apples to Oranges

For vehicles that use a liquid fuel like gasoline, diesel or even ethanol, the miles-per-gallon measurement, commonly abbreviated as "mpg," works well. At a glance, you can tell from a car's mpg rating whether it'll be an infrequent visitor at refueling stations or whether it'll guzzle like a group of frat boys visiting Cancun.

But for cars that don't use a liquid fuel, the term miles-per-gallon is pretty useless, since gallons (and liters) are a measurement of liquid. The problem isn't just with electricity. Take gases used for fuel, for instance. Units of compressed natural gas are expressed in cubic feet or cubic meters, depending on where in the world you happen to live. Now there's a measurement likely to confuse your average car buyer.

Electricity, when measured for the purpose of billing a customer, is expressed in kilowatt-hours. To put that in some context, ten 100-watt light bulbs running for an hour would equal one kilowatt-hour (kWh). While that does mean something, it's still not a very helpful measure if you're Tom Tirekicker, researching and trying to compare different vehicles' performance.

What we need is a measure that's usable for comparing vehicles with different fuel technologies. To do that, we must first figure out how much energy is stored in a gallon of gasoline and then convert that to electricity. The Environmental Protection Agency (EPA) has done the heavy lifting on that science problem already: According to the agency, one gallon of gasoline contains the same amount of energy as 33.7 kWh.

When you break it all down, our dear old friend miles per gallon measures the distance a vehicle can travel using a given unit of energy. We want to do the same thing with our electric-powered vehicles -- but by swapping out gallons for the more appropriate kilowatt-hours. It might sound complicated at first, but the many minds involved have managed to boil all that down to a quite simple, elegant formula:

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

Here's a real world example, the EPA's rating for the electric Nissan Leaf:

  • 100 miles driven X 33.7 kWh ÷ 34.0 kWh X 1 gallon of gasoline = 99 MPGe

OK, so E=MC2 it ain't, but still not bad for something designed to quantify a concept that's so complex.

Thus, you have a (reasonably) accurate, meaningful way to compare automotive apples to oranges (while leaving "automotive lemons" out of the mix, one would hope).

Clearly this is a treatment that will work well with electric vehicles. Just remember, the "e" in MPGe stands for "equivalent," not electric. Next we'll take a look at all the different fuel types to which this measurement applies.

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