ANN ARBOR, Mich. -- The EPA is preparing to tighten its oversight of fuel economy testing and reporting this year in an effort to align the ratings on vehicles' stickers more closely with their real-world capability.

A key area of focus will be the agency's "coast-down" test, which has been the source of mistakes that prompted Hyundai, Kia, Ford and Mercedes-Benz to relabel several models since 2012. During the test, vehicles are made to coast to a stop from 80 mph, generating readings on aerodynamic drag and friction in the drivetrain, among other data points. Those data are then used to program dynamometers to simulate the vehicle's behavior on real roads during laboratory testing.

By year end, the EPA will issue revised guidelines clarifying how automakers should conduct the testing, said Christopher Grundler, director of the EPA's Office of Transportation and Air Quality. The agency also plans to continue stepped-up audits of the automakers' tests, and may require manufacturers to verify their own preproduction testing results using production models.

The updated guidelines are intended standardize coast-down test procedures that Grundler says vary by automaker, and come after several high-profile cases of automakers having to lower fuel economy ratings because of internal and EPA audits that found errors in coast-down testing.

"Especially these days, when things are so competitive and customers are saying, 'This is my No. 1 purchase criteria,' it seems to me that [the EPA and the industry] both would want to have as much confidence as possible that the label value represents a good number," Grundler said in an interview.

Grundler says the EPA also will close a decades-old loophole that allowed automakers to test one vehicle and apply its mpg ratings to other vehicles that share powertrains and have similar weights. That loophole was at the center of a discrepancy that prompted Ford to sharply lower mpg ratings on its C-Max line in August 2013.

But the coast-down test has been most nettlesome in recent years. In 2012, Hyundai and Kia lowered the mpg ratings on some 900,000 units sold in the U.S. after the EPA discovered flaws in company coast-down test data, prompting an agency investigation and tighter oversight of the company's fuel economy ratings that has continued to this day. In June, Ford cut the mpg ratings again on the C-Max line and other models after it discovered errors in preproduction coast-down test data after an internal audit.

"We have been jawboning the automakers for two years now, saying, 'We really think you ought to be doing some audits of production vehicles to double check all the assumptions you made three years ago on your prototypes,'" Grundler said. "Ford does that, and that's how they discovered this error that's been corrected. Other firms don't."

Before the Hyundai and Kia relabeling, the EPA hadn't regularly audited coast-down test data for decades. Since then, it has conducted about 70 targeted audits using production cars.

That coast-down test errors have been found at just four automakers after some 70 audits is "actually pretty reassuring," Grundler said.

"We see no evidence of an industrywide systemic issue here," he said. "But we're paying attention because of what's at stake."