The EPA's Ancient System

The Environmental Protection Agency's fuel economy tests begin with data submitted by automakers based on their own internal testing conducted under controlled laboratory conditions using a standardized test procedure specified by federal law. EPA reviews the results and confirms about 10 percent to 15 percent of these with testing at its own laboratory.

The EPA's procedures for testing fuel economy are in need of updating. Pictured are the agency's test labs.

In either case, lab testing is done on a dynamometer following a standardized routine that simulates city and highway driving conditions — or so it seems. This protocol does not reflect today’s driving patterns because testing assumes an average speed of 48 mph and a top speed of 60 mph, little congestion, gentle acceleration and braking, no use of mpg-robbing air conditioning and only moderate ambient temperatures.

Ultimately, the EPA determines fuel economy numbers by measuring the amount of carbon in each vehicle's exhaust that’s captured during testing. The EPA says this is a more precise indicator of fuel consumption than a fuel gauge. As an acknowledgement that its testing protocol reflects ideal conditions and to avoid overestimating real-world mileage, the EPA began reducing city fuel-economy results by 10 percent and highway results by 22 percent in the 1980s. No additional adjustments have been instituted, or testing procedures modified, to reflect significant changes in motor-vehicle technology since then.

Until those procedures do change, it's best to keep in mind that disparities will exist and that differences in operating conditions, climate and driving habits can influence actual fuel economy considerably.