The Truth About EPA Mileage Estimates

The Environmental Protection Agency’s current Federal Test Procedure (FTP) for city mileage was originally designed to represent a typical trip on Los Angeles streets. The test– codenamed FTP-72– begins with a cold start from 70 degrees. It then runs for 7.5 miles at an average of 19.6 mph, with a peak speed of 56.7 mph (from a short freeway segment). The EPA Highway Fuel Economy Test (HWFET) starts with a warm engine, runs for 10.26 miles, averages 48.3 mph, and peaks at around 60 mph. Does anyone in the real world drive like that?

Obviously not. By the ‘80’s, the EPA was deluged with complaints that its gas mileage estimates were entirely unrealistic (i.e. hopelessly optimistic). In 1984, the EPA reacted by lowering estimates across the board to “more accurately reflect driving styles and conditions.” The government agency trimmed EPA window sticker stats by 10 percent for the city, 22 percent for highway. This overdue amendment helped consumers feel more confident about calculating their fuel costs (and driving skills).

Thanks in part to the huge discrepancy between the EPA’s fuel economy estimates for hybrid-powered cars and their real world mileage, test procedures have been refined again.

For ’08 model vehicles, the EPA numbers will provide “in use” figures based on a combination of the old tests and new tests. The new procedures will incorporate faster speeds (up to 80mph), greater acceleration (up to eight mph per second), warmer outside operating temperature (including air conditioner use) and colder outside operating temperatures (including heater and defroster use).

The industry fully (and rightly) expects the new testing procedures to lower EPA “in use” mileage estimates even further, especially for hybrid drive cars like the Toyota Prius that benefit from “gentle driving” in the city cycle. Meanwhile, we can still glean some interesting information from the EPA stats.

Although it’s not well known, the EPA keeps two sets of books for public perusal. Pistonheads can scan both the government’s official EPA numbers for all cars certified for the U.S. and the raw test results. But wait; there’s more! The EPA also provides a given model’s engine displacement, engine revs in top gear, inertia weight (test weight for the EPA test) and dynamometer resistance settings.

University of Michigan physics Professor Marc Ross has developed formulas that can crunch the EPA’s dynamometer resistance readings to compute the amount of drag in pounds for speeds between 45 and 55 mph. Working with his equations, a math-minded motorist can predict a vehicle’s EPA gas mileage under a variety of test and non-test conditions.

For example, the 2007 Avalon has inertia weight of 3875 lbs. At 50 mph, with its 3.5-liter engine turning at 1555 rpm, the Avalon generates 91 pounds of drag. Bottom line: the Avalon achieves 39.6 mpg on the HWFET test– as compared to 31 mpg on the window sticker.

The 2007 Five Hundred has a 4000 lbs. inertia weight. At 50 mph, with its 3.0-liter engine turning at 1520 RPM, the soon-to-be Taurus generates 105 pounds of drag. Bottom line: 37 mpg on the HWFET test– as compared to 29 mpg on the EPA window sticker.

Theoretically, the Five Hundred achieves 35.3 mpg at 55 mph, 30.4 mpg at 65 mph, and 22.6 mpg at 85 mph. The Avalon gets 36.2 mpg at 55 mph, 32.1 mpg at 65 mph, and 24.8 mpg at 85 mph.

The Avalon’s lower-profile aerodynamic shape and bigger engine make it more frugal than the Five Hundred. (Five Hundred owners pay a price for Ford’s decision to make the Volvo-based model tall and boxy to attract SUV buyers.) With its smaller engine, the Five Hundred is also slower than the Avalon– which has been slated for not being fast off the line. Both vehicles have tall gearing to aid fuel economy.

Toyota doesn’t use pixie dust or a magic carburetor to get better mileage than the Five Hundred. Toyota achieves better fuel efficiency because of the choices its creators made between looks, room, fuel economy, acceleration and drivability.

With revised EPA testing procedures leading to lowered fuel economy figures, with revised CAFE legislation mandating higher required fleet fuel efficiency averages, the pressure is on for automakers to produce more– and more popular– higher mileage vehicles. Only there’s a loophole.

According to the official EPA website, their new “in use” stats will “not affect the CAFE calculation for purposes of determining manufacturers’ compliance with the CAFE standard.” In other words, the new CAFE averages will continue to be based on the old EPA tests. Well how about that?