The change in mileage is not a surprise, given the controversy that has been swirling around the mileage estimates of the Ford Fusion Hybrid and C-Max Hybrid. Last December, Consumer Reportsreported it got just 37 MPG combined city-highway for the C-Max in real-world test conditions.
Since then, there have been other reports of C-Max owners reporting mileage below the estimate stated on the window sticker.
C-Max sales unlikely to suffer
The C-Max doesn't have big sales volume, with units sold totaling 23,040 this year. That represents less than 2 percent of Ford's U.S. sales year-to-date. Still, the car has been helping Ford gain traction in the rapidly growing hybrid market.
(Read more: Don't expect Ford Hybrid MPG dispute to slow sales)
"I don't think there will be long-term damage to the C-Max because of this mileage issue," says Karl Brauer, senior analyst at Kelley Blue Book. "The C-Max has sold well on the east and west coast, and I think that will continue."
In the first half of this year Ford sold 38,141 C-Max Hybrids and Fusion Hybrids, an increase of more than 700 percent compared with the first half of last year. While the Ford hybrid sales have been impressive, it still trails market leader Toyota, which sold more than 116,000 Prius models in between January and June.
Consumers Watching Mileage
Ford is not the first automaker that has been forced to restate mileage estimates for its vehicles.
Last Year, Hyundai and KIA admitted overstating the fuel economy ratings for approximately 900,000 2011-2013 models. That move came after the Environmental Protection Agency determined the Korean automakers' methodology for testing fuel efficiency was flawed.
Brauer says the move by Ford shows consumers are paying more attention to the mileage of their cars and trucks. "When cars don't get what's posted on the window sticker, people do notice. "
—By CNBC's Phil LeBeau. Follow him on Twitter
Questions? Comments? BehindTheWheel@cnbc.com.