It’s an eye-popping number: 230 miles per gallon. General Motors says that’s the mileage its new plug-in car, the Chevrolet Volt, will get in city driving, assuming it goes on sale as planned in late 2010.
It’s also misleading. The Volt will get astounding “gas” mileage because it won’t exactly be powered by gas. At least a lot of the time it won’t, if drivers use the car the way GM envisions. That’s the whole point of an electric car: To propel a vehicle with something other than petroleum. If the Volt were powered mostly by a windmill or a nuclear reactor, it would also get great gas mileage, since the fuel would be coming from some other source.
The 230 MPG figure is GM’s estimate, and it’s a good bet that the government will never make it official. GM knows this. The automaker has been working with the Environmental Protection Agency, arbiter of fuel-economy figures for American cars, on new ways to measure fuel economy for cars like the Volt that run on more than one fuel. Technically, hybrids like the Toyota Prius (which averages 48 MPG) fall into this category, but the MPG format still works for hybrids because the electric battery pack mainly assists the gas engine. For all the innovation in hybrids, there’s no power cord and the car basically runs on the same gasoline as every other car. It does so more efficiently, but its fuel economy can still be expressed effectively as miles driven per gallon of gas consumed.
The Volt and other alternative-fuel cars will be different. The Volt will have a huge battery pack charged through a power outlet. So a good chunk of its power won’t come from gasoline at all, but from the electrical power grid. Electricity is typically metered out in kilowatts, not gallons. We tend to measure electrical usage over time rather than distance, and we pay for it that way too, with rates based on a kilowatt-hour of electricity, for example. So in terms of energy usage, the Volt will be more like a refrigerator than an automobile. A refrigerator that cruises down the highway. To measure its efficiency, we’d really want to know how many miles per kilowatt it averaged. MPK. Or perhaps it would be kilowatts per mile, KPM.
But the Volt will only run on battery power until the battery's charge runs low on power. Then a gas engine will kick in, powering a generator that runs the car. (GM calls the engine an “alternative power source.” Get it? It’s the gas engine that’s the alternative….) GM says that under normal driving conditions, the battery will power the car for about 40 miles until it needs to be recharged. So if you never drive more than 40 miles between charges, your gas mileage will be the same as somebody on a bicycle—which is either 0 or infinity, I’m not sure which. But your MPK (or KPM) rating will be quite measureable—and hopefully better than a refrigerator.
My guess is that GM’s figure of 230 MPG is based on a driving profile that’s dominated by battery power. That’s probably why they only released a figure for city driving. Highway driving, or a combined average of city and highway, implies trips of more than 40 miles, at which point the Volt’s battery would run low on power and you’d start burning gas.
GM hopes you don’t do that, because the Volt’s performance will be less impressive when the gas engine is powering the car. The huge battery will amount to a lot of dead weight when it’s not in use, and we don’t know yet what kind of mileage the car will get beyond the 40-mile all-electric range. Buyers shelling out $40,000 for the Volt—the expected sticker price—might be underwhelmed by the car if they end up doing a lot of extended driving.
Burning gas also complicates the whole fuel economy equation, since a single trip could entail battery power drawn from an outlet, which is measured in kilowatts, along with gas from the pump, measured in gallons. The EPA needs to boil that down to a single set of numbers that doesn’t sound like a science lecture.
Then we have to figure out if this will all cost more or less than a car powered by gas. GM is sure it will cost less to operate the Volt than a gas-powered car, and the higher gas prices go, the less the Volt will cost by comparison. But how much less? GM says a 40-mile charge will cost about 40 cents at current electricity rates, which means you’d spend $1.20 to drive 100 miles. In a gas-powered car averaging a healthy 30 MPG, by comparison, you’d spend $10 in gas to go 100 miles, if gas cost $3 per gallon. That’s 8 times more costly than driving on the Volt’s battery power alone, but the real cost to drivers will depend on how much driving is powered by each type of fuel.
If we’re lucky, there will be more conundrums like this, since a lot of promising gasoline alternatives can’t be effectively described in terms of miles or dollars per gallon. Hydrogen, for instance, is usually measured in liters or kilograms, and some hydrogen-powered European prototypes measure their fuel economy in kilograms consumed for every 100 kilometers traveled. For Americans, that takes some mental adjusting, since a lower number is better—the opposite of miles per gallon. New kinds of hybrids and bi-fuel vehicles, like diesel-electrics and hydrogen-electrics, could further complicate the math if they ever get produced.
[See why GM is ready to rebound.]
Up till now, there have been just a few niche vehicles that required some workaround to measure fuel economy, and the government has typically relied on some kind of MPG-equivalent. The Honda Civic GX, for instance, which runs on natural gas, gets the equivalent of 36 miles per gallon in combined driving—even though natural gas is typically measured in cubic feet, not gallons. The EV1, GM’s first electric car, got the equivalent of about 60 miles per gallon. It stands to reason that the Volt will also be rated as an MPG-equivalent, although there may be a corresponding kilowatt-based measure we’ll have to start getting used to. Maybe the Volt will even earn an Energy Star rating, making it one of your most efficient appliances. And the only one with room for four.
Corrected on : This story was clarified to indicate that when the Volt's battery runs low, a gas engine will power a generator that propels the car, not provide power to the wheels directly.