Corporate Average Fuel Economy: How Automakers Rank

Catch any car commercial these days, and you'll probably get an earful on gas mileage. It seems every automaker has turf to claim: GM's commercials assert good gas mileage as part of the company's ethos, while Honda ran a Super Bowl ad claiming it's the most fuel-efficient carmaker in America.

Unfortunately, there is no gold standard to rank gas mileage among automakers. The best-known effort — though it is not without its detractors — is the government's Corporate Average Fuel Economy program, established in 1975. Each year, the Environmental Protection Agency calculates CAFE based on gas mileage estimates for each car in an automaker's lineup and the number of those cars produced that year. The lineups split into three groups: Domestic passenger cars, which includes cars built using mostly domestically sourced parts, from either an American or foreign nameplate; imported passenger cars, built mostly with parts from abroad; and light trucks, which includes everything from pickups and minivans to crossover SUVs, no matter where their parts come from.

Gas mileage figures come from automakers and go to the National Highway Traffic Safety Administration and then to the EPA for verification, so it can take until well into the following year for final CAFE numbers to be published. Based on NHTSA's preliminary data, here's where automakers stood for 2009:

Though CAFE ratings might look like an outright average of the gas mileage for all the vehicles in a carmaker's lineup, it's not that straightforward. The numbers on the window sticker and the ones reported to the government are "entirely separate," NHTSA spokesman Eric Bolton said. City and highway mpg figures come from the EPA's lab tests, with adjustments to replicate real-world conditions. CAFE figures, meanwhile, are derived using different testing procedures and have various incentives built in.

Models that can run on ethanol-based E85, for instance, are allowed mileage calculations based on the small fraction — 15 percent — of regular gas that's used when running on E85. That means E85-compatible SUVs like the Chevrolet Suburban or Dodge Durango could register gas mileage similar to an economy car. Mileage figures are averaged across a range of engines and transmissions per vehicle, too, so a particularly high- or low-mileage trim level could skew the numbers for an entire carline. Heavy-duty models, like the Ford F-Series Super Duty and Chevrolet Silverado HD, are exempt from the calculations.

Nonetheless, the program has enormous implications. Since the 1990s, Congress has imposed serious financial penalties on high-volume automakers that fall short of federally imposed standards — 27.5 mpg for cars and 22.2 mpg for light trucks. In May 2009, President Obama announced that fuel economy standards will increase to 39 mpg for all domestically sold passenger vehicles and 30 mpg for light trucks for 2016.