Relax, Your Mileage Will Still Vary…

You may have noticed that the 2008 Toyota Prius doesn't get the same great mileage that it did last year, even though Toyota hasn't made any major modifications to the vehicle. In fact, most 2008 models have dropped in average fuel economy. Combined Average Fuel Economy (CAFE) numbers will play a more important role in determining the "real-world" fuel economy of a vehicle in the coming years, as automakers struggle to raise their fleet average fuel economy to meet NHTSA goals for both fuel economy and carbon emissions.

The EPA is in the process of revising the calculations for fuel economy estimates, so you may notice that your favorite vehicles don't perform (at least on paper) as well as they used to. To confuse the situation even more, identical 2008 models on dealer lots may have two different fuel economy ratings. Another number also makes its appearance: the carbon footprint of a vehicle. The lower the number, the fewer tons of CO2 the vehicle emits on an annual basis.

The EPA's fuel economy ratings are based on an average of 15,000 miles per year, and the assumption that 45% of those miles are accumulated in highway driving conditions. The revised fuel economy estimates are a product of the EPA's desire to provide a more realistic estimate of the fuel economy a consumer can expect to receive. The EPA's method of measuring fuel economy is based on testing metrics that were developed in the 1960s and 1970s. The test standards did not take into account driver behavior like rapid acceleration, or the proliferation of accessories, like air conditioners, that can reduce average fuel economy.

To accommodate these common, real-world conditions, the EPA included three new metrics in their calculations. One new test includes rapid accelerations at speeds up to 80 mph. A second factors in air conditioners and other fuel-consuming accessories, and a third test measures the impact of a 20°F cold-start.

The EPA has revised all fuel economy ratings for all vehicles manufactured since 1985. The new rates appear on the EPA's web site. The EPA has also provided a link to the estimates calculated using the old formula. The EPA notes however, as always, that your mileage will still vary based on driving behavior and road conditions.

The EPA Web site provides side-by-side comparison tools, the ability to adjust fuel economy for your vehicle based on the type and amount of driving you do, and current fuel prices. The site also provides an "energy impact score" which computes the number of barrels of oil your car "consumes" based on your driving each year. Finally, the site provides an estimate of your out-of-pocket cost to drive the car 25 miles, and your expected annual fuel costs.

The tool provides a nice way to do side-by-side comparisons for all makes and models, from your old junker to the new model you're considering. It's also a good way to compare hybrid models to see the real differences between the hybrids and their conventional cousins.

Photo Credit: Dain Hubley