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Why MPGs for EVs Makes Sense
Why MPGs for EVs Makes Sense
Posted Thursday, October 1, 2009 - 4:33pm
According to USA Today, the EPA is planning to “rejigger wild mileage claims” for electric cars:
“The concept of judging fuel efficiency on electric cars by the ‘miles per gallon’ they consume is dying a slow death. The EPA and other government agencies are working on a formula that will accurately tell people how fuel efficient electric vehicles are—but since those cars don't use gas, miles per gallon doesn't make any sense.”
Um, why exactly? As EVs come out of the gate and enter the nation’s vast auto market, they’ll be competing directly with vehicles whose fuel efficiency is judged by MPGs. And as long as potential EV customers, and especially EV buyers who choose a gas-aided hybrid like the Chevy Volt, understand their cars’ all-electric, per charge range limit, then a direct MPG comparison is fair game.
The whole point of vehicles like the Volt and the all-electric Nissan Leaf is that they’re offering an alternative to the all-internal-combustion, all-the-time propulsion that even parallel gas-electric hybrids like the Prius provide. To create a separate category for them means created a competitive pool of very few vehicles. So sure, the Volt probably won’t get 230 mpg, as had been provocatively claimed by GM in ads, but everyone knows that.
This alternative to MPGs seems wacky, however:
“An Israeli firm last week urged the EPA to come up with a three-pronged number that would tell people how much electricity a car uses when it’s fully charged, how much electricity it takes to charge the car and how much gas the car uses when it’s out of juice.”
USA Today doesn’t name the Israeli company, but why bother creating such a complex number? If you’re in the market for an EV like the Volt that also burns gas as needed, you’re probably not all that worried about how much electricity you’re going to require to charge the car up (you’ve already crossed that threshold), and as for how much juice it takes, that’ll be an abstract amount, as most folks will want to know how long the charge will take. And that that’s an easy answer: overnight. Leaving us with...MPGs (!) when the car starts to use gas.
Long live MPGs, people. Let’s not throw out this perfectly useful, perfectly well-understood measurement. We know the EV startups and the big automakers getting into electric cars won’t want to. And of these two groups, the big automakers have the most to lose by undercutting their massive patent holdings on internal-combustion, gas-burning engines. GM doesn’t seem worried. So why should the EPA?
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Comments
No, they don't
First, MPG is neither 'perfectly useful' nor 'perfectly well-understood [sic]'. Because it is a reciprocal unit, the relationship between a car's MPG rating and how much it costs to run is non-linear. Not everyone realizes that an increase from 30 to 35 mpg will not save you nearly as much money as an increase from 20 to 25 mpg. The Europeans get this - they use litres/100 km, which is the right way up. (If this paragraph is not perfectly obvious to you on first reading, then I stand by my assertion that MPGs are neither perfectly useful nor perfectly understood.)
In writing "So sure, the Volt probably won’t get 230 mpg, as had been provocatively claimed by GM in ads, but everyone knows that", you are admitting that the MPG figure is completely meaningless, so how can you go on to defend it? The Volt figure could come out anywhere between about 40 MPG (its efficiency when running on the gasoline engine) and infinity MPG (what it gets when running on batteries), depending on the length of the EPA test cycle. In what way is this predictive of actual use, and how is a customer supposed to use this for comparison?
What matters to the consumer is how much it costs per mile to run the car. The problem with hybrids is that the cost is variable, depending on how far you drive. Now the EPA ratings for combustion engine cars already have two figures (city/highway), so presumably the public is capable of handling that. So I see no reason why the system couldn't be changed to two $/mile figures: one for a standard city cycle, and one for a standard highway cycle. This could be based on average prices for gasoline and electricity (much like the Energyguide figures for appliances). I doubt that the testing methodology would have to change all that much.
This would apply to any car, regardless of fuel source. Hybrids would use only the new figures. Gasoline-engined cars would use both for a while until everyone was used to it. The transition would be simple and would get us most of the way to providing a reasonable basis for comparison, regardless of powertrain technology. I cannot imagine why this would not be a desirable outcome.