For some <a href="http://christianlouboutin-pascher.weebly.com" rel="nofollow">soldes louboutin</a>    other hybrids, the variation is greater. Fueleconomy.gov lists reports from 65 owners of the new Ford C-Max hybrid, who have together averaged 39.1 miles per gallon in combined <a href="http://2013coachoutlet.webs.com" rel="nofollow">Coach Factory</a>    city and highway driving鈥攏ot the 47 mpg that Ford advertises based on the performance the car recorded in the fi <a href="http://longchamppaschersoldes.weebly.com" rel="nofollow">sac longchamp pliage</a>   ve-part fuel-economy test program required by the EPA. Among those drivers reporting, some achieved as much as 56 mpg, and others as few as 28 mpg.EPA officials are in the process of getting a C-Max to test themselves at the agency's fuel-economy laboratory in Ann Arbor, Mich. They want to study the vehic <a href="http://tomsshoes-outlet.jimdo.com" rel="nofollow">toms outlet</a>   le, in part, because it presents a new technology wrinkle that may be flummoxing its long-standing tests. The car is designed to go as fast as 62 miles per hour using batteries alone. The EPA's traditional highway test is conducted at an average speed of 48.3 miles per hour and a top speed of 60. The agency in 2008 added new tests designed to narrow the gap with real-world driving b <a href="http://borse-louisvuitton-outlet.webs.com" rel="nofollow">Louis Vuitton Sito Ufficiale Borse</a>   y simulating highway speeds up to 80 mph, as well as driving in hot and cold weather. It is also considering conducting tests using gasoline cut with 10% ethanol, like most commercially available fuel, instead of the pure gasoline prescribed now. The 10% ethanol blends tend to drop mileage by 3% to 4%, the agency says.A 2011 study by two Oak Ridge National Laboratory researchers concluded that the results of the EPA's 2008 testing system "tend to underestimate the real-world benefits of fuel economy."  Raj Nair, Ford's vice president for engineering, says Ford is confident that it has conducted the EPA tests properly on all its vehicles. But especially with hybrids such as the C-Max, he says, variables such as temperature, a driver's behavior and the engineering choices Ford makes to balance mileage and performance can make a big difference.Ford could design its throttle systems to deliver slower, steadier acceleration and get higher scores on the EPA tests, Mr. Nair says. "We choose not to do that," he says, because Ford wants a car that is "fun to drive."Rick Larrick, a professor at Duke University's Fuqua School of Business, says consumers would be better served if the government and auto makers simply junked the miles-per-gallon measurement altogether. Mr. Larrick says gallons consumed over 100 miles of driving鈥攖he way mileage is measured in Europe鈥攊s a more useful yardstick because, ultimately, the variance in mpgs has less impact than most people think.A car that gets 33 miles per gallon consumes 3 gallons per 100 miles. How much would you save by trading for a car that gets 50 mpg? The answer: 1 gallon per 100 miles. "You see these large differences in miles per gallon that don't make much difference," says Mr. Larrick.A version of this article appeared January 30, 2013, on page D1 in the U.S. edition of The Wall Street Journal, with the headline: Why Your Car's Mileage May Not Always Measure Up.