From MPG to EPM: Plan for "Electricity Per Mile" Ratings Takes Hold

MPGForget about General Motors and Nissan’s triple-digit MPG claims for upcoming plug-in vehicles. “Electricity per mile” will be the shorthand of choice for the fuel efficiency of electric cars, if the Society of Automotive Engineers has its way. As CNET’s Martin Lamonica reports this morning, the industry group, whose recommendations will likely be considered by the Environmental Protection Agency as it finalizes a rating system for plug-in vehicles, wants to see “electricity per mile” join mile-per-gallon ratings on fuel-economy stickers.

The proposal could bring automakers’ recent sky-high claims — 230 MPG for GM’s Chevy Volt, 367 MPG for the Nissan LEAF — down to earth, giving consumers a relevant and realistic shorthand for how much they would end up spending on electricity for different vehicles rather than forcing next-gen cars into the old mold of autos that run on gasoline.

Labeling vehicles with ratings in terms of “gallons per mile” or gallons per 10,000 miles has had backers among Duke University researchers for some time. The idea is to put fuel consumption and the costs of fuel front and center for new car buyers. Electricity per mile — more than miles per kilowatt-hour or miles per gallon-equivalent  — could have the same effect. The ratings could let consumers estimate fuel costs by simply multiplying the local electricity rate by the number of miles they usually drive and easily compare the costs of various vehicle models.

The upcoming generation of plug-in vehicles, which includes cars running exclusively on electricity as well as models like the Volt that will consume both electricity and gasoline, requires an expanded rating system, and SAE plans to finalize its recommendation for the EPA within six months. As the National Renewable Energy Laboratory’s Jeff Gonder (working on the SAE committee) explained to CNET, “There are two different fuels that are being used so you need to report what the usage is for those two fuels…If you combine them into one (number) artificially, you can’t derive a final output like annual costs” or greenhouse gas emissions.

Emission measurements present another challenge as the government works to update its rating system. As we explained in our cheat sheet on MPG claims for electric and hybrid cars, the simulation that the EPA currently uses to come up with city and highway MPG and emission ratings leaves much to be desired for plug-in vehicles. To start, estimates are based on the amount of emissions generated during the simulation, yet all-electric vehicles don’t have tailpipe emissions. NREL is now working on a new formula for the simulation to better reflect real-world performance of plug-in vehicles.

The sooner the auto industry and government settle on these rating systems and start giving consumers realistic expectations, the better it will be for the nascent electric vehicle market as it looks to move from the early-adopter stage to the mass market. In some cases, overhyping a company or tech that ultimately falls short of expectations can cause skepticism and distrust in an industry that could otherwise be quite promising.

Image courtesy of Flickr Creative Commons.

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