How Your Car Compares to the Chevy Volt

Forget mpg. A new metric is needed to compare gas-powered cars to GM's plug-in and other futuremobiles.

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General Motors has delivered some momentous news this year. There was that inconvenient bankruptcy filing on June 1, and the taxpayer bailout that now totals $51 billion. To get our minds off that, GM recently announced that the Chevrolet Volt, which is now basically a taxpayer-funded science project, will get the equivalent of 230 miles per gallon in city driving. Whoa. In overall driving, GM assures us the Volt will shatter the mythical 100-mpg mark.

There's reason to be skeptical, since GM has overpromised many times before. And if GM ever needed some favorable press, it's now.

But so far the Volt is standing up to scrutiny, and the closer we get to the scheduled launch late next year, the more innovative the Volt appears. Even though most Americans won't buy one, the Volt could create one of the first meaningful alternatives to gas-powered cars. It could also change the way we think about fuel economy—and reinvigorate beleaguered GM.

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The Volt isn't a hybrid like the Toyota Prius or Honda Insight, which rely on a battery pack to assist the gas engine that powers the car. The Volt is designed to run on battery power alone for up to 40 miles, with drivers able to recharge it through an ordinary electrical outlet. Once the battery runs low, a 4-cylinder engine will automatically start up to power a generator that will power the wheels. The reason the Volt could get the equivalent of 230 miles per gallon is that on many short trips, the gas engine wouldn't even be on.

I'm usually suspicious of impressive-sounding numbers, so I interviewed Tony Posawatz, who runs the Volt program for GM, to find out what the catch might be. The 230 figure isn't an official government number, as GM has readily acknowledged. The Environmental Protection Agency, arbiter of official fuel-economy figures, hasn't yet settled on a final method for measuring fuel economy for cars like the Volt that run on more than one type of fuel, with efficiency dependent on the length of the trip.

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Without providing an exact methodology, Posawatz explained some of the assumptions backing the 230-mpg figure: Drivers get just one charge, and most drive less than 40 miles on their city voyage, which is consistent with government data showing that most Americans drive less than 40 miles per day. Those people are "infinity drivers" who would literally burn no gas, ever, if they drove the Volt less than 40 miles every day (and they remembered to plug it in every day when they got home, and the Volt worked perfectly, and the car wasn't packed with weight-lifting equipment, and . . .). Some other drivers in GM's city profile presumably drive more than 40 miles or are horribly inefficient drivers and burn gas. That would lower the Volt's city gas mileage from infinity to 230 mpg, or the equivalent.

Highway mileage, Posawatz admits, will be lower. But overall, he says, "it will be better than any conventional car out there." That means the Volt would handily beat the Prius, which gets a combined 50 mpg and is the reigning gas-mileage champ among ordinary production cars. That alone would be a victory for GM.

But forget mpg. That's so 1990s. The EPA prefers to measure the efficiency of electric plug-ins in terms of kilowatt hours per 100 miles traveled. GM says the Volt's optimal performance will be about 25 kWh/100 miles. Boy, that sounds efficient, doesn't it?

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Actually, who knows? So forget kilowatt hours, too. The Volt and other energy breakthroughs will probably force us to become more conversant in electrospeak, but for now there's a simpler way to compare the efficiency of the Volt to just about every other car: Cost per mile. "That's the all-in," says Posawatz. The beauty of this metric is that anybody can calculate comparisons, with info that's generally available, in terms we all understand.

GM says that 100 miles of battery-powered travel in the Volt will require about $2.75 worth of electricity. That's based on typical electricity rates of about 11 cents per kWh. (Check your electric bill to find out how much you pay.) That amounts to 2.75 cents per mile. If drivers charge their Volt overnight, when off-peak electricity rates are much lower, the cost could be as little as 1 cent per mile, according to Posawatz.

GM hasn't said what kind of "mileage" the Volt will get when it's running on the 4-cylinder engine, but somebody who regularly depleted the battery and kept on driving might pay 4 cents per mile or more, after factoring in the cost of gas. (Of course, it's possible that a Volt owner might never charge the battery and always drive on gas, which would be foolish but not that much different from an individual who buys a big, off-road SUV and rarely drives more than a few miles from home.) Overall, Posawatz suggests that 2.5 cents per mile might be a good comparison figure.

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Let's assume he's overstating the Volt's efficiency, and use 4 cents per mile. How does that compare to my Mazda 3? Here's the math: The version of the 3 I own averages 25 mpg. Gas currently costs $2.62 per gallon where I live, so it costs me $10.48 to drive 100 miles. If I divide by 100, I get a cost per mile of about 10.5 cents.

Hmmm. That doesn't seem so good compared with the Volt, and the 3 is pretty thrifty. If gas spiked to $4 a gallon again—which many energy analysts think is quite possible, by the way—my Mazda would average about 16 cents per mile. The Volt, meanwhile, would probably still cost 4 cents per mile or less, since electricity prices tend to be more stable than gas prices.

For other popular cars, the comparison is worse. A four-wheel-drive Toyota Highlander, for example, averages 19 mpg, which means the gas required to go 100 miles costs about $14, at $2.62 per gallon. That equates to 14 cents per mile. With $4-a-gallon gas, it would be 21 cents per mile. At the other end of the spectrum, fueling the Prius costs a little over 5 cents per mile at $2.62, and 8 cents per mile at $4.

Anybody can play with these numbers, and even customize them for their own region, for both the Volt and the family car. Gas prices for various regions and some states are here. The Department of Energy breaks down average electricity costs by state at this Web site, for people who want to plug in local prices to determine the Volt's potential charging costs where they live. For now, we're dependent on GM's claim that the Volt uses about 25 kWh of electricity to go 100 miles. So people doing their own calculations need to figure out how much 25 kWh of electricity would cost, then divide by 100 to get the cost per mile. GM does appear to be playing it straight in terms of the gas and electricity prices it's using.

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There are a lot of other factors that will ultimately determine whether the Volt is worth the expected sticker price of about $40,000 and whether it actually cuts fossil-fuel use when the entire "well-to-wheel" costs, in terms of both dollars and pollution, are calculated. The cost-per-mile figures I've been fiddling with are for fuel only. Maintenance, repairs and other factors could end up costing more on the Volt than on other cars. Or the Volt's huge battery might not be able to deliver a full 40-mile driving range over its entire lifespan in real-world conditions.

The Volt's higher price could also outweigh any gain in fuel efficiency, although government rebates will probably bring the cost down by several thousand dollars. Still, the Volt is a first-of-a-kind product, which means the technology costs aren't expected to be recouped right away. And so far, GM appears to be delivering on its promises. One of those promises is to deliver a game-changing vehicle. If the Volt catches on, we could start to see window stickers and official government Web calculators that help consumers compare the cost-per-mile of various vehicles, no matter what fuel they run on. That could start the fade-out of mpg, which sounds like a new game to me. And a pretty exciting one, too.