Washington — The Environmental Protection Agency said Monday it is issuing new guidelines to automakers for conducting fuel economy testing. The move comes after major automakers have had to restate miles-per-gallon ratings for certain models.

The guidelines details how vehicles must be prepared before being tested — including what is an acceptable level of wear on the tires and how vehicles should be broken in prior to testing.

Also at issue are “road load” tests used to determine the impact of aerodynamic drag and tire rolling resistance on gas mileage. Currently, that is measured at 50 miles per hour. Under the new guidelines, automakers must measure the results at all speeds up to 70 mph.

Last year, the agency said it was considering writing new formal regulations covering mileage labeling by automakers. Instead, it is opting to issue the guidelines. It typically takes EPA two to three years to write new legal binding mandates.

Chris Grundler, the EPA’s director of the Office of Transportation and Air Quality, said in an interview that “it’s a very dynamic time” in the auto industry, and the agency doesn’t want to spend several years crafting rules that could be outdated when issued.

“Writing regulations takes time,” Grundler said. “When you are working in the rapidly changing environment that we’re in right now, we want to make sure that we are agile enough and flexible enough to change with those times.”

He said the new guidance is aimed at getting better results. He noted that EPA will follow the guidelines when it conducts audits of automaker testing. “Manufacturers will know exactly how we are conducting this testing,” Grundler said.

Karl Brauer, senior analyst at Kelley Blue Book, praised the new guidelines: “We’ll probably never know if automakers were intentionally or incidentally pushing the boundaries of fuel efficiency testing, but the new guidelines will make it harder for a car company to claim ignorance if its advertised numbers don’t align with real-world results.”

The EPA has hefty tools if automakers do overstate fuel economy ratings.

In November, Korean automakers Hyundai Motor Co. and Kia Motors Corp. agreed to a record-setting $360 million settlement for overstating fuel economy ratings. The largest-ever settlement includes a $100 million civil penalty, forfeiting $210 million in greenhouse gas emission credits and spending $50 million to ensure independent auditing of its current and future vehicles.

In August 2013, Ford Motor Co. dropped mileage ratings for the 2013 C-Max hybrid from a combined 47 mpg to 43 mpg. In June 2013, it lowered mileage ratings on six cars, including combined city/highway rating on the Lincoln MKZ hybrid by 7 mpg. Ford agreed to drop the fuel economy rating on the 2013 C-Max hybrid crossover from a combined 47 mpg to 43 mpg — a nearly 10 percent reduction — and will compensate owners for the worse-than-promised fuel economy.

Under the EPA’s rules, in place since 1977, Ford was able to assign the same mileage rating to the C-Max as the Fusion hybrid sedan, which also has a 47 mpg rating, because they are in the same family of vehicles. While the protocol has worked for conventional vehicles, it has not been as effective for hybrids, officials said.

In a statement in 2013, the EPA said it would look at rewriting rules for hybrid systems that are used across multiple vehicles “to ensure that consumers are consistently given the accurate fuel economy information on which they have come to rely.”

The EPA may drop plans to issue a new regulation to close the loophole that’s been in place for more than 30 years. Grundler said the agency may be able to achieve the same outcome through other strategies.

Automakers are watching this issue very closely after several major restatements. “Without question, industry is paying more attention — and that’s a good thing,” Grundler said.

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