Lower gas mileage numbers don't mean thirstier carsCar buyers shopping for fuel economy this year should expect to see some unexpectedly low mpg figures on 2008 models, some of which are hitting showrooms now. Don't get all stressed. New cars have not suddenly become thirstier.

Starting with 2008 models (which went on sale as early as January 2, 2007), the methodology for measuring and reporting fuel economy has been reworked to make published miles-per-gallon (mpg) ratings more accurate. The trouble for consumers is this: 2007 models and leftover 2006 vehicles on the lot next to the '08s will appear to have superior fuel economy. But they don't. This conundrum exists because 2007-and-earlier models use an Environmental Protection Agency (EPA) fuel economy measurement that is being phased out.

Your mileage has varied For years, consumers have groused that their new car simply can't match the gas mileage estimates displayed on the window sticker or featured in advertisements. The typical gut reaction has been to blame the carmaker, attributing the shortfall to an overenthusiastic marketing department. We see this type of comment all the time in our forums, and it seems that no company — even hybrid car manufacturers — is immune.

What consumers may not know is that the U.S. government, via the EPA, specifies all aspects of fuel economy ratings and how the mpg tests are carried out, including the "City" and "Highway" driving patterns. Those patterns are simulated on machines called dynamometers (dynos) to make the results accurate and repeatable. EPA's influence even penetrates to the design of the window sticker itself, right down to the point size and fonts used. So if the numbers seem too "optimistic" compared to real-world results, the manufacturers themselves aren't necessarily guilty as charged.

The reason why fuel economy estimates have been coming out too high is simple: The EPA-specified testing and reporting method has not been updated since 1985. Since then, a lot has changed. For one thing, the former national speed limit of 55 mph has been abolished. So instead of topping out at 60 mph, the new highway rating test includes speeds up to 80 mph.

And then there are added scenarios the old tests never included. Things like very hot and very cold temperatures and driving with the air-conditioner and defroster on are now part of the testing regimen. Acceleration rates in the old tests were funeral-procession sedate, owing to the limits of the dynos used when the tests were originally conceived in the '70s. More spirited, but realistic, acceleration and braking rates are now specified.