For the last 100 years, calculating a car's efficiency was easy, one determined how many miles a car could travel under certain standardized conditions on one gallon of gasoline.

However, the new era of electric and partially electric cars is turning that convention on its head.

This was too brought to a head earlier this month when GM announced that Volt would get 230 MPG in typical city driving, and later that day Nissan tweeted that its LEAF would get 367 MPG.

In the case of the Volt, that number was arrived at by looking at the average amount of driving a cohort of city drivers did, presuming they were driving Volts and charging daily, and determined a total of 230 miles of driving would be covered over time for every one gallon of gas used.

Nissan's number was based on a completely unrelated measure of the petroleum equivalence factor, which converts electric consumption into an equivalent amount of fossil fuel.

These numbers aren't the end, but the beginning of a potential list of numbers that if displayed on window stickers may make it hard for consumers to compare and choose vehicles.  Some even argue that a dollars per mile measurement should be adopted, but due to volatility of energy prices and wide variability among different energy sources this method is insufficient as well.

Progressive Automotive X Prize is a marketing-neutral organization which will award $10 million to the winner of a competition among vehicles for the one that will get at least 100 MPG.

They are promoting the widespread adoption of a measure known as mile per gallon equivalents (MPGe), determined by the following formula:

MPGe = (miles driven) / [(total energy of all fuels consumed)/(energy of one gallon of gasoline)])

This system essentially levels the playing field for all energy sources propelling the car including electricity, gas, or alternative fuels.

As examples, using these methods, the Tesla Roadster consuming 53 kwh over 244 miles of driving would get an MPGe of 158. The Nissan LEAF traveling 100 miles on 24 kwh of charge would get 142 MPGe.

And what about the Volt?

Well the calculator found here would work for it too, but still missing is the value for the Volt's MPG in charge-sustaining mode, Another problem is the fact that MPGe will vary as a function of range, from 170 MPGe for trips under 40 miles down to 58.2 MPGe on a 200 mile drive (assuming 50 MPG in generator mode.)

I asked John Shore who is Senior Advisor of Progressive Automotive X PRIZE what total driving distance he believes the Volt should be analyzed over.

He wrote:

Two relevant data points are the MPGe achieved at the 100 mile and 200 mile range requirements of the Alternative and Mainstream Classes, respectively. But it’s very important to note that the overall MPGe value used for scoring our competition will likely be considerably higher than these values, since MPGe over shorter ranges will contribute more to the overall value. Thus you might want to estimate the Volt’s MPGe at the various trip lengths listed in the table on page 36 of the Competition Guidelines – current version available here . You could then estimate a combined MPGe by taking a weighted average with the distribution weights shown in the table.
If we do the math he suggests up to 100 miles (99% of trips), the Volt then gets 167 MPGe, beating both the LEAF and the Roadster.

[NOTE: Graphic above is from Wall Street Journal article and illustrates yet another efficiency measurement, gallons per mile]