GM's 230 MPG Rating Is Unrealistic

chevy volt real tbi

Will wonders never cease in cleantech communication?

GM announced today that the EPA has given the GM Volt a MPG rating on the city cycle of 230 miles. Sounds great, right?

The problem is that anytime you try to get to “miles per gallon” using vehicles that don’t necessary use “gallons” you end up in a very strange debate with very strange outcomes.

Rational people can argue that to determine the MPG equivalent of an electric vehicle you simply take the energy equivalent of a gallon of gas (33.7 kWh) and determine how far the electric vehicle would go on that energy. A fairer way to measure this is to count energy coming out of the source, since there are losses in the charging process (forget for a minute losses during transmission and the whole issue of how electricity is generated)

It is by this calculation that you get Tesla’s claim that the Roadster is about 135 MPGe. The battery is 53 kWh. Assuming 15% loss on charging the total energy used on a full charge is 62.3 KWh. That is equivalent to the energy in 1.85 gallons of gas. Take the EPA combined range of 244 miles on a full charge and you get about 132 MPGe. You get different numbers if you use a different drive cycle or if you don’t account for charging losses.

So is the Chevy Volt almost twice as efficient as the Tesla Roadster? Hardly. If you apply the logic that the EPA seems to have applied in assigning the rating then the MPG of the Tesla Roadster, or any EV for that matter, would be infinite.

It seems that in assigning the rating, the EPA agreed with GM’s argument that the rating should reflect the average daily driving routines of a US driver. Most days, the average person doesn’t even drive 40 miles, so they would not exhaust the battery range of the Volt, therefore using no actual gasoline. They still use energy from the grid, however, but that doesn’t seem to have been accounted for. Given some kind of assumption of daily driving patterns, with the occasional long distance drive, the EPA came to 230 miles per gallon of actual gasoline.

So it isn’t an “untruth”, as long as the main thing you are concerned about is the burning of gasoline as a resource, as opposed to the actual energy efficiency of the system. The problem is that this number will be broadly discussed as a comparison to other cars, particularly the Prius. People will improperly conclude that the Volt is about 5 times more efficient that the Prius, which simply isn’t true.

In my opinion a fairer (but still flawed) approach would be the following:

Assume the energy in gallon of gasoline will charge the Volt’s 16 kWh battery and still have 53% of the liquid gasoline remaining (16/33.7 = .47). If you take the remaining fuel and run the on board generator after the battery has been depleted (assume you get 40 miles electric as GM has said). You need to assume a certain effeciency of the genset directly running the car. From what I have heard a generous assumption would be 50 MPG while running on the genset. The remaining fuel would get you 26.3 miles down the road. The total miles on that one gallon of gas is 66.3 miles by that approach.

The problem with this approach is it just doesn’t sound that impressive, and one can argue that it isn’t accurate either because people don’t drive an average of 66 miles per day. This is one reason GM argued the other way.

But why would GM argue to the other extreme of the spectrum, inviting scepticism, instead of looking for a reasonable middle ground? The answer to that is very simple: increasingly stringent CAFE requirements.

P.S> Since I am at the Plug-In 2009 conference today, I will share my analysis with the GM folks and ask what they think.


This post originally appeared on Darryl Siry’s blog.



DISCLOSURE: CODA Automotive, an EV Manufacturer which plans to bring an all-electric sedan to market in 2010, is a client of mine. This blog is representative of my independent opinion on this issue, and not necessarily the perspective of CODA.

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