Why MPG Ratings are Lower for '08

It's no secret that real-world gas mileage is often far different from what's listed on window stickers and in car commercials. That's because cars are tested for gas mileage in laboratories, under conditions that don't replicate the high speeds, hilly terrain and heavy loads most drivers encounter.

To try to close the gap between the lab and the real world, the Environmental Protection Agency has mandated that automakers start using new tests that do account for real-world conditions like higher speeds, faster acceleration, air conditioner usage and cold weather. The changes will be implemented beginning with the 2008 model year.

Integrated into the existing tests, the new procedures should result in significantly lower ratings. We checked out the adjusted numbers for 10 cars, from hybrids to SUVs, and found that they decrease anywhere from 6 to 16 percent from their 2007 equivalents. Hybrids and other gas-sippers will see the highest drops because their smaller, more-efficient engines typically have to work harder to achieve higher speeds, faster acceleration and other new test criteria.

Despite the lower numbers, 2008 cars will be just as efficient as they were under the old ratings. The new ratings are simply an effort to close the gap between advertised figures and real-world mileage.

The 2007 Chevrolet Impala's mileage rating is predicted to drop to 18/28 city/highway in the 2008 model year because of new testing standards; actual performance will not change.

The 2007 Chevrolet Impala's mileage rating is predicted to drop to 18/28 city/highway in the 2008 model year because of new testing standards; actual performance will not change.

For the next few years, the EPA will allow automakers to use a standardized adjustment factor to calculate the new mileage figures for their vehicles instead of mandating that the cars be retested, EPA spokesman John Millett said. The same calculation will tell you what a pre-2008 model's mileage would be under the new tests. Click here to see.

Here are some examples:

MPG Comparisons
Below, mileage estimates for 2007 models are compared to estimates using the new 2008 regulations.
Vehicle2007 city/hwy mpg2008 city/hwy mpg
Ford Expedition 5.4L 2WD14/2012/18
Honda Ridgeline 3.5L 4WD16/2115/20
Chevrolet Impala 3.5L21/3118/28
Toyota Prius 1.5L60/5148/45

This isn't the first time the EPA has adjusted its ratings system; the most recent adjustment was in 1987. Even with the changes, the 2008 guidelines won't be perfect. The new criteria don't account for everything — they assume flat terrain, no towing and roughly 300 pounds of weight inside the car, including all occupants. Driving with roof racks, using four-wheel drive, idling for long periods and a number of other factors can reduce real-world gas mileage even further. Check out Cars.com's Tips for Saving Fuel for pointers on mileage-friendly driving.

Posted on 3/28/07