This list ranks the combined average fuel economy for every fleet of 2008 models sold by major premium brands — 13 total. We excluded companies that sell fewer than 5,000 vehicles in the United States annually. Hummer didn't make the list because its H2 SUV is exempt from the Environmental Protection Agency's fuel-economy requirements for passenger vehicles.

To devise our own corporate average fuel economy ratings, we averaged the mileage estimates published in the EPA's 2008 Fuel Economy Guide for all variations of each model a luxury automaker sells. We computed city and highway mileage separately for each brand. From those city/highway averages, we calculated a combined miles-per-gallon rating for each nameplate using the EPA's formula that puts more significane on city mileage.

Each model variation is weighted equally no matter how many are sold. And we treat cars and SUVs alike. Therefore these averages do not equal the brands' official CAFE figures, and comparing them to the 27.5 mpg (car) and 22.2 mpg (light truck) standards set by the NHTSA isn't recommended.