Ford C-Max Hybrid gas mileage revised. EPA to update rules industrywide.

Ford is the latest in a string of auto companies that have scaled back mileage claims, after customers complained that the 2013 Ford C-Max Hybrid did not live up to its fuel economy label.

Cars are not what they were 40 years ago, but some gas-mileage rules are.

That has prompted the Environmental Protection Agency (EPA) to rethink how it rates the fuel economy of new cars. Traditionally, the EPA has allowed companies to use the same fuel-economy rating for cars with the same engine, transmission, and weight class.

But today's highly efficient hybrid cars are more sensitive to minor design differences, even when the core of the car is virtually identical. That can cause discrepancies between estimated and actual mileages, which rankle consumers looking to save on gas costs.

Ford is the latest in a string of auto companies that have scaled back mileage claims. It acted after customers complained that the 2013 Ford C-Max Hybrid did not live up to its label values of 47 miles per gallon (m.p.g.) for combined highway and city driving.

Ford based its rating for the C-Max on the test done for the Ford Fusion Hybrid, which has the same engine, transmission, and test weight. But the C-Max has a bulkier design than the Fusion, analysts said, which could account for the difference in their actual mileages.  

EPA announced Thursday it would revise that number down to 43 m.p.g. combined. Ford is voluntarily revising its labels and refunding $550 to customers who have purchased the vehicle.

“This is yet another sign that real-world fuel economy has to match manufacturers’ claims,” Karl Brauer, senior analyst at Kelley Blue Book, said in an e-mailed statement. “Ford isn’t the first automaker to run into trouble here, but with today’s heightened level of accountability, maybe they’ll be the last.”

Last November, automakers Kia and Hyundai revised mileage claims on about 900,000 vehicles combined after the EPA announced it was investigating overstated mileage claims. Label values were revised downward by an average of 3 percent – from 27 to 26 mpg. 

Fuel economy ratings have taken on greater significance as automakers increasingly compete to satisfy federal efficiency standards and appeal to eco-friendly, budget-constrained car buyers. There's been pressure on EPA to revise the standards, analysts said, and the agency announced Thursday it "will be working with consumer advocates, environmental organizations and auto manufacturers, to propose revised fuel economy labeling regulations to ensure that consumers are consistently given the accurate fuel economy information on which they have come to rely."

It's unclear how or when to expect changes to gas-mileage labels, but it could one day lead to "one set of tests for cars with pure internal combustion engines and another set of tests for hybrids and all-electrics," said John O'Dell, senior editor for fuel efficiency and green cars at Edmunds.com, a car-shopping website based in Santa Monica, Calif.

"They're trying to do it all with one test cycle and you just can’t," Mr. O'Dell added in a telephone interview.

Even with a perfect system, analysts stressed that there will always be discrepancies between how a car performs on the test track and in the real world. Fuel-economy ratings serve as a good benchmark for comparing between vehicles, but drivers shouldn't be surprised if expectations aren't always met. 

"The EPA rating has become sort of a gospel," O'Dell said. "It’s not gospel, but we’re tending to look at it as gospel."  

Weather, road conditions, tire pressure, and a host of other factors can significantly affect the efficiency of a car. One of the most important variables is how aggressive a person drives, analysts said.

"Drivers can control their fuel economy by accelerating a little more slowly and taking their foot off the gas sooner, rather than braking early," John Nielsen, managing director of engineering at national auto club AAA, said in a telephone interview. "It adds up to big savings."