The Truth About EPA Mileage Estimates

[The Straight Story]

The Truth About EPA Mileage Estimates

By Paul Milenkovic
April 24, 2007 - 17,744 views

newepasticker.jpgThe 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.
....

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.

http://www.thetruthaboutcars.com/the-truth-about-epa-mileage-estimates/

[pleah]

1. K&R Very interesting. n/t