By Mike Ramsey 

Ford Motor Co. lowered the mileage ratings on six 2013 and 2014 models, mostly hybrids and plug-in electric hybrids, and said it would pay owners as much as $1,050 to compensate for underestimating fuel costs.

It was the second time within a year that the Dearborn, Mich., auto maker has admitted to overstating the mileage ratings on its vehicles. The company said each time overstatements resulted from errors conducting government-prescribed tests.

Hyundai Motor Co. and Kia Motors Corp. admitted in 2012 they had overstated the fuel economy of more than a third of the vehicles sold in the U.S. for the previous two years. The car makers also agreed to reimburse customers, and blamed the overstatements on testing errors.

One of the Ford vehicles, the C-Max hybrid, already had its mileage rating lowered. The latest rating has fallen to a combined, city/highway rating of 40 miles per gallon, down from 43 mpg. When the C-Max was launched, Ford advertised it with a combined rating of 47 mpg.

The rating change lets the air out of Ford's efforts to trumpet its vehicles' fuel economy, particularly over Toyota Motor Corp. When Ford launched its Fusion hybrid in late 2012, the company said it far outstripped the fuel economy of the rival Camry hybrid, which has a combined mileage of 41 mpg. Now the Fusion hybrid's 42 mpg rating is one notch higher.

The rating changes affects all of Ford's hybrids, including the C-Max plug-in hybrid and Fusion plug-in hybrid as well as most 2014 model-year Fiesta subcompact cars.

Ford said it would compensate more than 200,000 customers who bought these vehicles with payments of ranging between $125 and $1,050 depending on which vehicle and whether it was leased or purchased. The company wouldn't provide an estimate of the total cost of the payouts.

The biggest restatement affected the 2013 and 2014 Lincoln MKZ hybrid sedan. Previously advertised at 45 mpg in combined driving, the car will be sold with a mileage rating of 38 mpg city, 37 highway. People who bought the car will get $1,050 back from Ford.

"Fuel economy is very important to customers and to us. This was our mistake, plain and simple. We've corrected our error and we are taking steps" to prevent it from happening again, Raj Nair, Ford's global product development chief, said during a conference call.

Ford discovered the latest mistakes and reported them to the Environmental Protection Agency, which regulates fuel economy testing. The EPA said in a statement that it oversaw Ford's re-testing of the vehicles, and conducted independent tests. The agency and Ford said they have agreed to improved tests for future vehicles, under EPA supervision.

Mr. Nair said Ford didn't pay fines to the EPA for the mistakes.

"This issue highlights the need for continued strong oversight of the fuel economy labeling program," said Chris Grundler, director of EPA's Office of Transportation and Air Quality in a statement. "Consumers need to trust that fuel economy window stickers are giving consumers reliable and fair estimates of real world fuel economy."

Ford said the payments would come from the company's warranty reserve, but that the issue wasn't connected to the company's $400 million increase in the reserve announced during first quarter earnings.

Ford wouldn't estimate the cost of the payments to customers, which was based on a calculation of the difference between the advertised rate and the change.

AutoPacific Inc. analyst Dave Sullivan said the Ford move could influence other auto makers to be more conservative with their fuel economy estimates. Hybrid vehicle buyers are more sensitive about fuel economy than buyers of conventional cars, he said. "A couple hundred dollars or some sort of monetary value isn't going to be able to buy someone's trust."

Ford said the error occurred in the calculation used to determine something called the "total road load horsepower," a resistance measure on a machine called a dynamometer that is used to test vehicle performance in a static, lab setting.

Mr. Nair said the correlation of wind-tunnel data to the computer model was incorrect, creating too little resistance. That led to a big disparity in results, particularly for the heavier hybrid vehicles, such as the Lincoln MKZ hybrid, which had its fuel economy reduced to a combined 38 mpg from a combined 45 mpg.

Ford said it confirmed the problem in its testing in March, notified the EPA, and proceeded to retest its entire lineup of vehicles using a new and modified formula.

Write to Mike Ramsey at michael.ramsey@wsj.com

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