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What is MPGe: Electric Car Fuel Economy Ratings Explained

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Ford recently announced the EPA fuel efficiency rating for its electric Focus model. Since it doesn’t burn any gasoline, the number isn’t in miles per gallon (MPG), but was given as miles per gallon gasoline equivalent, or MPGe. A new term to the automotive lexicon, it’s worth exploring exactly what MPGe means and how an MPGe rating is determined, especially as the number of electric cars and plug-in electric hybrids on the roads continues to increase.

Why use MPGe?

In order to compare the fuel efficiency of various vehicles, MPGe had to be introduced. The rating first came into use with the fully electric Nissan Leaf and the Chevrolet Volt, which offers a plug-in option to deliver anywhere from 25 to 40 miles of electric range before the gasoline engine kicks in to charge the car’s batteries, which continue to power the electric motor and the wheels.

Since electric vehicles don’t use gas, the traditional miles per gallon measurement wouldn’t make much sense. The most appropriate measurement would be Kilowatt Hours (kWh) since that’s the way electricity usage in houses is measured. However, using kWh makes it difficult to compare electric cars to non-electric cars. That’s why MPGe was created, to be able to compare the fuel efficiency of electric cars to other vehicles. MPGe is also an appropriate unit of measurement for other alternate fuels, like hydrogen, or compressed natural gas.

How do we get MPGe numbers?

In order to compare gas and electricity consumption, a common unit that measures energy is required. Burning one gallon of gasoline creates 115,000 BTU. The electrical equal to this is 33.7 kWh. Therefore, the distance an electric vehicle can travel on 33.7 kWh of electricity is comparable to how far a conventional internal combustion vehicle can travel on one gallon of gasoline.

Comparing MPGe of electric vehicles.

Comparing the MPGe of Electric vehicles is possible, so let’s take a look at how they stack up. For the Volt, Fisker Karma and Prius PHEV keep in mind that these vehicles are equipped with a gas powered range extender. This means we can use gas MPG numbers for part of the EPA rating.

Fisker, only gave us one MPGe number, the combined one. Tesla would not give us MPGe numbers of their roadster, or upcoming Model S, or Model X vehicles.

Where does MPGe fall apart?

The major problem with MPGe is that it doesn’t say exactly what electric car owners need to know. Range is probably the most important number that electric car buyers will want to look at. While range is present on the EPA sticker label, it’s just not as prominent as MPGe. As you can see in the chart above, MPGe has no relationship on range. Electric vehicles really depend on battery size to determine range, so that would be another key specification prospective buyers should take a look at.

As more and more electric vehicles come onto the market, this should start to overshadow the MPG numbers. Additionally, MPG numbers are losing trust with consumers. Some manufacturers are using the more generous (by roughly 25%) CAFE numbers instead of actual EPA test numbers which can further mislead customers.

With electric vehicles only making up 1% of the total cars purchased, the MPGe number plays a greater role, helping consumers to compare with conventional cars. As more electric vehicles enter the market, MPGe numbers will be less relevant, and buyers can look range as a defining factor in buying their cars.