Methodology

Whether using new numbers or old, EPA fuel-economy estimates remain the best way to rank fuel winners and losers. That’s because no other organization measures the fuel efficiency of as many vehicles as the EPA does. Even if the agency’s fuel-mileage figures don’t conform to yours, they enable you to accurately compare one car’s efficiency to another. Think of them as comparative measures, not predictions.

In our case, the current fuel-economy ratings enabled us to compare the efficiency of every 2007 production vehicle sold in the U.S. (except those that the EPA deems to be “special purpose vehicles”). To rank them, we averaged the individual city/highway ratings published by the EPA into single-figure, combined ratings. The formula, provided by the EPA, assumes that city driving accounts for 55 percent of a vehicle’s mileage, leaving 45 percent to highway travel.

In cases where models come with a choice of manual or automatic transmissions, we averaged the separate fuel-economy ratings for each option. We also averaged in the ratings of convertible models for the few exotics on the least fuel-efficient list that offer open-top versions.

To estimate annual fuel costs, we assumed each vehicle travels 15,000 miles a year — the national average used by the EPA and others. Dividing that by each vehicle’s combined mpg rating told us how many gallons the car would consume during that 15,000-mile annum. Then we multiplied the number of gallons by the price per gallon for each vehicle’s appropriate fuel grade.

We used U.S. average fuel prices current as of early November 2006, published by AAA at fuelgaugereport.com. For regular gas, that was $2.209, a 25 percent reduction from the national average of $2.818 when we looked at 2006 models about one year earlier. Premium gasoline averaged $2.431, or 27 percent below its year-earlier average. Of course, the estimated annual fuel costs will change as pump prices change. What’s more, it’s not likely that the high-priced, high-powered, exotic cars in the gas-guzzling class will log 15,000 miles per year. Still, our estimated annual fuel costs give you an equitable measure for comparing the cars.

To determine the 10 most fuel-efficient and 10 least fuel-efficient vehicles, we started with the efficiency ratings in the 2007 Fuel Economy Guide, available at fueleconomy.gov. We used the version published at the official start of the model year, posted by the Environmental Protection Agency in October 2006.