Gas Mileage: Why MPG Numbers Really Matter

Enlarge Image Image Gallery Monitoring your fuel economy and becoming more aware of what influences it is one of the easiest and most fulfilling ways to make a difference for the environment.  ISTOCKPHOTO/RYAN BURKE

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When Berkeley, Calif., resident Bradley Berman drives his 2006 Toyota Prius around town, the fuel economy indicator pushes into the low 40s. That’s a far cry from the number advertised for the gasoline-electric hybrid, so Berman’s disappointment is understandable.

He says, “When you see 60 mpg on the sticker for city driving, and you’re basically doing city driving but getting in the low-40s, you think ‘That’s terrible!’” Berman’s numbers improve on longer trips, but they still come up short compared to the official miles per gallon estimate for the Prius. Berman should know. As the founder and editor of HybridCars.com, he hears from numerous hybrid drivers across the country who share similar experiences.

For better or worse, hybrids have become the poster child for vehicles not living up to fuel economy expectations. That may be because hybrid drivers are theoretically more interested in saving oil and thus complain louder; because hybrids display real-time mpg results and thus their drivers are more aware of the results; or perhaps simply because, as the reigning mpg champ, they’re an easy target.

The truth, however, is that advertised fuel economies of all vehicles, including conventional models, have been slowly but steadily drifting from reality for more than two decades.

To remedy these inaccuracies, the U.S. Environmental Protection Agency (EPA) recently changed the way it calculates fuel economy estimates. The new math will first affect the numbers for model year 2008 vehicles. The results will be lower, more realistic city and highway mpg estimates that “help us capture the way Americans are driving now,” says EPA spokesperson John Millett.

Here’s a little perspective to help understand why the EPA made these changes. In the 1970s, the Agency developed tests designed to mimic city and highway driving conditions in a laboratory. The test results were posted on new vehicles, giving consumers valuable information during the ’70s energy crisis.

The results were also used for new fuel-efficiency requirements known as Corporate Average Fuel Economy, or CAFE. The federal standards, which still exist, require automakers to meet an average mpg for all cars and trucks they sell. While automakers can produce inefficient vehicles, they must also sell more efficient models to even out their fleet’s total average.

In 1985, the EPA began to reduce the results of its mpg equations by about 15 percent when it became apparent the tests no longer reflected real-world driving. Among other things, the tests did not include the use of air conditioning and speeds greater than 60 mph. Yet over the two decades since then, even more has changed. Today’s vehicles are substantially more powerful than those of yesterday and, as a result, people are driving more aggressively. They’re also driving faster, enabled by increases in highway speed limits across the nation. Vehicle amenities have also changed. For example, 20 years ago, energy-thirsty air conditioning was optional on vehicles; today it is all but ubiquitous. Because each of these changes lowers fuel economy, tests that neglect to factor them in give artificially inflated numbers.

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