General Motors' upcoming Volt plug-in gasoline-electric hybrid has already grabbed plenty of buzz over the past couple of years. But Volt mania reached a fever pitch this summer when General Motors announced that it would achieve 230 miles per gallon.

Later the same week, Nissan revealed that its upcoming Leaf electric vehicle would achieve 367 mpg.

Before you go put your money down, beware-neither of those two numbers are official U.S. Environmental Protection Agency (EPA) fuel economy estimates. The EPA hasn't yet formalized any method of testing or rating plug-in hybrid vehicles or a new generation of electric vehicles, and you won't find either of those numbers in the EPA's annual Fuel Economy Guide or on window stickers because they're still prototypes.

Turns out both of these triple-digit numbers rely on optimistic methodologies that are favorable for the respective vehicles but not or easily applied to other types of vehicles or compared to present-day fuel economy figures.

The recent claims remind John DeCicco, a transportation and energy issues expert, of the time just after the 1973 oil crisis and before the first official EPA fuel economy ratings were available, in 1975. Some manufacturers around at that time advertised their vehicles with "outlandish mileage claims," recalled DeCicco, also a senior lecturer on energy and climate policy at the University of Michigan. "We've again got a situation where manufacturers will spin the number to look good," he said.

Looking at the 230-mpg figure, there's no way of saying simply that the Volt would use less than a quarter of the fuel of a 50-mpg Toyota Prius to go the same distance.

It is however fair to say that in typical driving a 50-mpg Prius will go twice as far on a gallon of gas as a 2010 Toyota Corolla XRS, rated at 25 mpg. Such comparisons are fair game for any light vehicle that's officially on sale in the U.S. Every one carries EPA fuel economy ratings, which are designed to give prospective owners an idea of relative fuel cost in real-world driving as well as a way of comparing various models on equal ground.

The ratings are listed in the EPA's annual Fuel Economy Guide, at, or on the window sticker of any new vehicle; they're also used in figuring an automaker's corporate average fuel economy (CAFE) and in assessing a gas-guzzler tax on some vehicles. They've become an important way in which shoppers can easily compare one vehicle to another, and it assures a level playing field because of rigorous testing.

Running on fumes-on a treadmill

In the tests, vehicles are driven under controlled conditions in a laboratory using a dynamometer-described by the EPA as a treadmill for cars. The dynamometer accounts for things like drag and air resistance, and temperature, humidity, and even barometric pressure are all accounted for.

Nearly all vehicles are tested by the manufacturers according to strict stipulations, with full test results sent to the EPA; then the EPA confirms between 10 and 15 percent of the results annually at its National Vehicles and Fuel Emission Laboratory in Ann Arbor, Michigan. According to Linc Wehrly, the manager of light-duty vehicle testing at the laboratory, vehicles are chosen for confirmation either randomly, or because they have new technology, are a new entrant in the market, or boast especially high or low fuel economy.

The two basic test cycles, City and Highway, have been run in much the same way for decades. The City test represents low-speed and stop-and-go driving, with a cold start, a top speed of 56 mph, and an average of just 20 mph, with 23 stops over 11 miles. Maximum acceleration is an extremely gentle 3.3 mph/second though. The Highway test, is intended to replicate free-flow highway driving, with an average speed of 48 mph and a top speed of 60 mph over 10 miles, with no stops and no idling.

Over the years the two sets of test numbers have been adjusted a couple of times to better match the numbers seen in real-world driving. Most recently, in 2008, along with a new fuel economy label for new cars, three new tests were added-High Speed, AC, and Cold Temp-to bring City and Highway estimates more in line with what a high percentages of drivers could obtain in real-world driving. Pretty self-explanatory by name, the three tests factor in more realistic rush-hour traffic congestion and speeds up to 80 mph; a test with the air conditioning on (it's turned off for the others); and a run of the City test cycle at 20 degrees Fahrenheit.

At first hybrid owners cried foul because the new system applied a more severe correction to vehicles getting higher mpg figures.

Regulators decided to use the new five-cycle tests "to recognize that there are different effects in different types of vehicles," confirmed Michael Love, the company's national regulatory affairs manager, who explained that the new fuel economy tests will have slightly more impact lower-power vehicles and smaller-displacement engines suffering bigger hits because of the AC test, for instance.

From 2008 until 2011, as the new tests are being phased in, manufacturers can choose from performing all five of the new tests to yield revised City and Highway figures, or adjust their figures from existing tests using a set of equations. From the 2011 model year on, manufacturers will be required to run all five test cycles, so don't be surprised if the numbers change a bit next year, too. Most major automakers have phased in the new method for some but not all of its vehicles this year.

"The biggest hurdle was telling consumers that there was no change in the technology of the vehicle" despite the different numbers for the last round of changes in 2008, said Charles Territo, communications director for the Auto Alliance, an association representing eleven different automakers.

"The regulations try to find a good middle ground," assessed Territo. There's no question that there are certain vehicles that fare better than others in the test, according to Territo, but the tests and ratings assure that overall vehicles are regulated to the same point.

Ramping up for major change

With a host of plug-in hybrids, electric vehicles (EVs), and extended-range EVs all expected to go on sale over the next several years, the agency needs to move quickly to put a framework in place for fairly rating these vehicles.

"That GM ad put the heat on them," said Love.

The EPA has in the past rated EVs (such as General Motors' EV1, which sold in the late '90s) in terms of kilowatt-hours per 100 miles, which typically yields a double-digit number somewhat like miles per gallon.

Most of the rest of the world is accustomed to comparing vehicles by consumption (lower is better, of course). already allows users to switch to a consumption-based figure, in gallons per 100 miles, if that helps them more easily compare vehicles.

Auto Alliance's Territo says that a consumption standard is certainly on the table for new standards, but with new units like kilowatt-hours thrown into the mix, it's not likely. "For years we've gone by mpg and that's what consumers understand," he said.

Indeed, the EPA has already been listing some alternative-fuel vehicles, like the natural-gas-powered Honda Civic GX, in gasoline-equivalent mpg.

Currently the EPA is working with manufacturers, the Society of Automotive Engineers (SAE), the State of California, the Department of Energy (DOE), and other groups to form a new standard for estimating and labeling fuel economy for vehicles with more than one energy source.

Labeling vehicles with two different efficiency ratings-which shoppers would weigh differently depending on how they intend to use a vehicle-is an option, but it would be tremendously confusing to most.

"There is an unambiguous way of doing this," and produce a new method that would be fair for all vehicles, argues DeCicco, who thinks that the best, most objective metric would be energy (possibly in BTUs) per mile.

Cost per mile would of course make the most sense to economically minded buyers, but it would become confusing and difficult to compare from year to year because of unstable energy prices.

And the challenge for plug-ins would be making it scientific, fair, and rational, no matter what the vehicle. For instance, the current fuel economy tests even allow for the state of charge in the batteries of a hybrid vehicle. With the battery packs in plug-in hybrid vehicles and extended-range EVs all different sizes and providing ranges from just 15 or 20 miles up to 60 miles or more in some prototypes, how would that be recognized without penalizing those with shorter range?

The EPA is working on that, and currently accepting comments on these possibilities. This will all be part of the next round of fuel economy standards phased in beginning in 2012, to yield the federally mandated 35.5-mpg fleet-wide average by 2016.

GM might have been a little premature in releasing its lofty figure for the Volt, but they highlighted one important point in the process: The next time you go shopping for a new car, those numbers on the window sticker could be very different.

Read More About the Chevy Volt

- GM Says Chevy Volt Will Get 230 mpg
- Chevy Volt Photos
- Electric Cars Available Now

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