Ford's internal testing process revealed an error in the EPA as it estimated city and highway mpg of six vehicles. The company is issuing corrected window stickers to dealers for all affected vehicles, and providing goodwill payments to vehicle owners.

The error in the original testing for the cars resulted in discrepancies in the estimated mpg. Most of the vehicles are having their EPA ratings reduced by between one and five mpg. The Lincoln MKZ Hybrid is being hit particularly hard, with its combined rating reduced from 45 to 38 mpg. The error affected all of Ford's 2013 and 2014 hybrid vehicles, as well as the 2014 Fiesta.

Ford will be distributing goodwill payments to the approximately 200,000 owners of affected vehicles based on the difference between the revised EPA rating and that originally advertised. The payments range from $125 for those who leased a 1.0L Fiesta to $1,050 for owners of the MKZ Hybrid.

"We know we won't always be perfect," says Raj Nair of Ford in a video announcement. "But we promise you that when we see an issue, we'll address it, and do all we can to make things right for our customers."

EPA testing is conducted using a dynamometer that simulates different speeds and road conditions. Since the test is conducted in a lab, engineers create a model that accounts for wind resistance and other drag that the vehicle would experience under real driving conditions. This is usually more accurate than a road test, which introduces many variables that can't be controlled.

The model, known as "Total Road Load Horsepower" or TRLHP, is unique to each vehicle, and is based on real-world testing. This includes a "coastdown" test that measures how long it takes for the vehicle to coast to a stop after driving a certain speed on a track.

The error arose from a recent change in how Ford incorporated data from wind tunnel testing into the TRLHP. Although the model appeared to be sound, Ford's routine internal testing revealed a discrepancy between the mpg originally recorded and that experience during road tests. The company rechecked all of its vehicles and found six that had been affected by the error.

As per Nair, the error has since been fixed. New window stickers displaying the revised EPA ratings will be distributed to dealers within the week, and Ford will not ship any more cars until the sticker information is corrected.