Because the issue of discrepancies between advertised EPA mileage ratings and real-world mileage affects a number of manufacturers, especially of hybrid gasoline-electric vehicles, the U.S. Environmental Protection Agency on Thursday said it planned to update its fuel-economy labeling regulations, in part to address those issues.

The EPA said the 2013 Ford C-Max would be relabeled under its new ratings system, to reflect estimated fuel economy of 40 miles per gallon on the highway, 45 mpg in the city and 43 mpg combined. Previously, the C-Max was rated at 47 mpg highway, city and combined.

Ford said it will send checks to C-Max customers to make up the difference in fuel costs - $550 to owners and $325 to lessees.

The EPA said it tested the C-Max after receiving consumer complaints that the vehicle did not achieve the label values of 47 mpg.

The move is a blow to the second-largest U.S. automaker, which has touted its superior fuel efficiency and been vocal in its desire to overtake Toyota Motor Corp as the leader of the U.S. hybrid segment.

But Ford's dilemma has implications for other manufacturers of hybrid vehicles, many of which have a reputation for falling well short of their advertised mileage.

Ford's announcement comes roughly 10 months after an EPA investigation showed that both Hyundai Motor Co and its affiliate Kia Motors Corp overstated their fuel economy by at least a mile per gallon.

General Motors, which sells a variety of hybrid vehicles, including the Chevrolet Volt, on Thursday said it will "work closely with the EPA on an improved process that can help deliver consumers reliable information backed by data."

Consumer Reports magazine said last year that the Ford C-Max and Fusion hybrids fell about 20 percent short of their promised fuel economy in road tests.

The automaker has faced several class action lawsuits that allege it overstated the gas mileage on its hybrids. Last month, it offered a free software update to boost the C-Max and Fusion hybrids' real-world economy.

Ford engineering boss Raj Nair said Thursday that under EPA rules the C-Max previously was lumped in the same family as the Fusion Hybrid, so both shared the 47 mpg rating. The 2014 C-Max, which is being tested separately, will received some hardware upgrades to improve its mileage.

The EPA on its website says its fuel-economy tests are "designed to reflect 'typical' driving conditions and driver behavior," but that its ratings "may not accurately predict" a vehicle's real-world mileage.

(Reporting by Deepa Seetharaman and Paul Lienert; Editing by Richard Chang and Ken Wills)

By Deepa Seetharaman