Understanding MPGe(Miles Per Gallon Equivalent)

Starting with 2013 model years, part of the NHTSA & EPA joint ruling adopted a common standard to compare fuel consumption of alternative fuel & advanced technology vehicles. This common standard is called the MPGe. MPGe stands for "Miles Per Gallon Equivalent". It means the kilowatt-hours of electricity, kilograms of hydrogen or cubic-feet of compressed natural gas that is equal to energy in one gallon of gasoline.

Understanding MPGe
In the US, we are accustomed to expressing energy units in miles per energy unit - mpg(miles per gallon). This is different than how we express the efficiency for alternative & advanced technology vehicles. For alternative/advanced technology vehicles the unit is expressed in energy unit per 100 miles. Expressing efficiency in energy unit per 100 miles is quite common in Europe. For us in the US, this is an alien way of measuring efficiency & hence to make it convenient for the masses, the EPA, in addition to providing the energy unit per 100 miles(ex: kw-hours/100 miles) also provides the easily recognizable MPGe. Note that this is a mpg equivalent & not the true mpg. This unit of efficiency is reserved for advanced & alternative technology vehicles. Note that this is an equivalent mpg of unconventional fuel & not the mpg of hybrid vehicles relying solely on conventional fuel.

MPGe on EPA Stickers
When you go to buy a new hybrid, alternative or advanced technology vehicle, the new EPA guidelines mandate that the EPA Window Sticker express the efficiency in MPGe. For example, for the 2014 Honda Accord Plugin Hybrid you will see a combined EPA efficiency of 115 MPGe listed in bold on the sticker. Below it you will also see 29 kilowatt-hours/100 miles listed. Per the formula devised to convert kilowatt-hours/100 miles to MPGe, you can easily see that 29 kw-hours/100 miles is 115 MPGe - See calculator here

Does this mean that EV's now give outrageous gas mileage
Ah! You see 115 MPGe, damn that is beyond belief & you start thinking how much you will save over the course of the year buying the latest hybrid, EV, alternative/advanced tech car. Before you do that, try to understand what MPGe is & isn't. MPGe is the mpg of the vehicle on an alternative source of power expressed as mpg. It is not the mpg if you had one gallon of gas in the car.

How is MPGe calculated
Lets take the example of the 2014 Accord Plugin. On electric power it gives 29 kilowatt-hours/100 miles. 1 gallon of unleaded gasoline is equivalent to 115,000 BTUs(British Thermal Units). A BTU is the amount of heat generated by burning & when you burn 1 gallon of unleaded gasoline it'll generate 115,000 BTU. Similarly, to generate 115,000 BTU, you will need to expend 33.7 kilowatt-hours of electricity. Comparing these 2, 1 gallon of gasoline is equivalent to 33.7 kilowatt-hours. But, the 2014 Accord Plugin utilizes 29 kilowatt-hours to go 100 miles, meaning we divide 33.7 by 29 & the multiple the result by 100 to arrive at the ~115 MPGe for the Accord plugin.

So I can still travel 100 miles
And here's the catch. The MPGe can be misleading since it is expressed per 100 miles. It is simply a unit of efficiency & not of the range. Hence, the EPA sticker also specifies the range for EVs & alternative/advanced technology vehicles. The range for the 2014 Accord Plugin is 13 miles. What this means is that on all electric power, the car will only travel 13 miles.

What is MPGe good for
MPGe is good when comparing EV'S & alternative/advanced technology vehicles. It is meaningless to compare the MPGe with the mpg of conventional fuel based cars.

When looking at MPGe, understand that it is only a unit of efficiency & pay attention to the range provided as that will make comparison easier to understand.