“What will fuel-economy numbers look like on the window stickers of electric vehicles coming to the US market next year?” asks Paul Weissler, in his recent article for the Society of Automotive Engineers. Trying to find the answer could short-circuit your brain.

The big numbers on the window sticker for BMW’s Mini E—currently in the hands of hundreds of consumers in a test program—are 33 in the city and 36 on the highway. But those are posted in kilowatt hours per 100 miles. The smaller text explains that the equivalent is 102 mpg city and 94 mpg on the highway.

When plug-in cars hit the US market in the next year or two, consumers will need a lot of help deciphering the efficiency figures of vehicles that carry electric fuel by the kilowatt hour rather than liquid fuel by the gallon. Nissan’s upcoming yet-to-be-named electric car, according to some tests, will get 367 miles per gallon. The Tesla Roadster is reported to get 135 miles per gallon. And the Chevy Volt plug-in hybrid…that depends.

If the EPA uses tests designed for electric cars to evaluate the Chevy Volt, the ratings could exceed 100 mpg. But if the government agency classifies the Volt as a hybrid and tests it as such, the EPA rating would drop to about 50 mpg. The difference could mean success or failure in the marketplace. Corporate Average Fuel Efficiency and sticker numbers for plug-in hybrids, which use gasoline and electricity in various degrees and ways depending on the specific vehicles design, have not yet been determined.

Plug-in Car Driver Recharging His Car

It will take time for consumers to learn a new system for evaluating the efficiency of cars that use little or no gasoline. That system has not yet been fully developed.

Today’s conventional hybrid gas-electric cars faced similar hurdles soon after they became popular. Some owners were disgruntled when they didn’t achieve the level of the Environmental Protection Agency numbers—such as the second-generation Toyota Prius’s initial EPA city rating of 60 miles per gallon. In 2006, the city and highway numbers were adjusted down to the 40s to more accurately reflect real-world driving. But the shift to powering cars from the grid will bring unprecedented levels of uncertainty and confusion.

Weissler explains that convention vehicles including Prius-style hybrids are derived from EPA dynamometer emissions lab tests—and then adjusted according to various legal mandates. For all-electric cars, the CAFE calculation begins with converting gasoline’s energy content into electrical terms—based on “multiplying factors” for the efficiency of electricity. The Department of Energy slightly increases the resulting CAFE number because it doesn’t directly use petroleum, but if the car uses any “petroleum-fired accessory, such as a heater,” the number is brought back down. All tests apply a 55/45 weighting on numbers derived from city test cycles rather than highway driving.

There are also inconsistencies between methods and calculations used by the EPA and the California Air Resources Board. The Mini E’s equivalent numbers are based on the CARB’s “petroleum energy content-equivalent” of 32,600 watt hours per gallon, rather than the EPA’s petroleum equivalent of 33,705 watt hours per gallon.

The Mini E’s EPA city and highway numbers averaged out to 98 mpg—but the fate of Nissan’s upcoming electric car is still uncertain. Weissler writes: “If the Nissan-measured weighted average is 223.57 W·h/mi, dividing 82,049 W·h/gal by 223.57 W·h/mi would equal 367 mpg.” Easy, right?

Finding a way to provide easy-to-grasp efficiency numbers to the non-technical general population of car buyers will be difficult. But it’s one more essential task necessary for adoption of cleaner greener electric-drive vehicles.