Unlimited mpg? The great Volt debate

Volt with vehicle line director Tony Posawatz

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NEW YORK (CNNMoney.com) -- How do you measure the fuel economy of an electric car? Is it the equivalent of 80 miles per gallon? 8,000?

Turns out it's a thorny issue - and General Motors is currently in long-term discussions with the Environmental Protection Agency to develop a standard.

The decision could make a big difference for GM (GM, Fortune 500), which is expected to unveil the Volt this week. The automaker's first long-range electric car could hit showrooms by 2010. The compromise between GM and the EPA could have a major impact on how effectively the Volt is marketed.

Discussions are still a long way from resolution, but progress is being made, said GM spokesman Kyle Johnson. "This is a very analytical and a very thoughtful process," Johnson said.

The EPA will say only that it is "still developing its policy for testing, measuring, and reporting fuel economy for plug-in electric hybrid vehicles."

Fuel economy for hybrid vehicles like the Toyota Prius is displayed in the same way as it is for any other gasoline-powered vehicle. It gets 46 mpg, for example, versus 19 mpg for a V-6 Ford Mustang.

That standard works because all the energy used by the Prius ultimately comes from burning gasoline. The Prius just uses that energy more efficiently than other cars do.

The Chevrolet Volt, on other hand, will get its power from two sources - a battery as well as a gasoline engine. How much of each depends largely on how far the car is driven.

The Volt's lithium-ion batteries will hold enough juice to drive the car for about 40 miles, GM has said. Once the car goes beyond that, a small gasoline engine will turn on, generating electricity to power the wheels.

When gasoline is providing the power, the Volt might get as much as 50 mpg. Since the gas engine is only generating electricity and because the battery will still have enough charge leftover to provide additional power for passing and merging, the Volt will still be very efficient even when running on gas power.

But that mpg figure does not take into account that the car has already gone 40 miles with no gas at all.

So let's say the car is driven 50 miles in a day. For the first 40 miles, no gas is used and during the last 10 miles, 0.2 gallons are used. That's the equivalent of 250 miles per gallon.

But, if the driver continues on to 80 miles, total fuel economy would drop to about 100 mpg.

And if the driver goes 300 miles, the fuel economy would be a just 62.5 mpg.

One way to perform a these tests has been suggested by the California Air Resources Board, which will have to test these cars to ensure compliance with that state's strict emissions standards. The CARB tests are based on a set of standards created by the Society of Automotive Engineers in 1999, anticipating just this sort of challenge.

"Right now, this is definitely the most complex testing we've ever done - by far," said John Swanton, an air pollution specialist with CARB.

CARB has proposed testing the cars twice.

Once, the cars will be run through a standard set of laboratory "driving" tests on a fully charged battery. These tests will only be used to verify electric-only driving range.

A second test will be with the battery drained, forcing use of the gas engine for the entire time. That will give CARB engineers a "worst case scenario" picture of tailpipe emissions.

CARB will use both tests to determine the emissions and fuel economy.

If CARB uses the SAE's standards then, based on the Volt's all-electric range, its fuel economy would be roughly double the "worst case scenario."

So if the Volt were to get, say, 50 miles a gallon while operating solely with its gasoline engine, the fuel economy rating would be 100 mpg.

Even so, that would be highly theoretical, much more so than for other cars. A Prius gets about 45 mpg in city driving no matter how far you drive it. The Volt's fuel economy estimate would apply only in one specific scenario.

GM's solution: Embrace the complexity.

A fuel economy window sticker proposed by GM in an April 2008, presentation to federal regulators shows fuel economy measured three ways: City and highway miles per gallon, city and highway miles per kilowatt hour and city and highway range (electric-only and total range.)

Assuming car shoppers know their local electrical rates, that information should allow them to judge how much the Volt would cost them to drive compared to other cars.

The trend with federal regulators like the EPA and NHTSA has been to simplify information for consumers, though, not make it more complex. In this case, that will be hard to do while still providing something meaningful.