If you drive a Ford C-Max or a Ford Fusion Hybrid, you might want to double check the fuel economy of your car. Two of Ford’s hybrids, the C-Max and the Fusion, fell short of fuel economy targets when tested by Consumer Reports, said the Detroit automaker on Friday. But Ford isn’t shying away from this potentially embarrassing issue, saying that it wants to work with the Environmental Protection Agency (EPA) to improve the way it determines fuel efficiency, claiming that the current testing methods leave too much to chance and produce erratic results.

Both the Ford C-Max and the Fusion are rated at 47 miles per gallon for city, highway and combined. But according to Consumer Reports, the C-Max only reached 37 miles per gallon overall, with 35 in the city and 38 on the highway. The Fusion didn’t fare much better at 39 miles per gallon, 35 in the city and 41 on the highway. Owners of the two vehicles have confirmed the lower performance, reporting 39.5 miles per gallon overall for both the C-Max and the Fusion.

Ford blames the EPA’s testing methods for the its poor fuel economy results. The tests, Nair says, don’t account for certain variables that have a significant impact on the results. For example, when an owner drives at 75 miles per hour instead of 65 miles per hour, it costs 7 miles per gallon. And if temperatures are below 40 degrees Farenheit, it lowers the fuel efficiency by 5 miles per gallon. Moreover, break-in miles can decrease fuel performance by 5 miles per gallon as well. Unaccounted-for details like these can add up to a 17-mile difference and that is why Ford is looking to work with the EPA to see if it needs to change the way it tests hybrids. “We absolutely agree with the EPA that hybrids are far more variable in the test cycle compared to real-world driving conditions in conventional vehicles. We’re working closely with the agency to determine if any changes are needed for the industry relevant to hybrid vehicle testing,” says Nair.

But most other hybrid car manufacturers don’t seem to have a problem with the EPA’s current testing methods because according to Consumer Reports, more than 80% of the vehicles tested by the magazine have been within 2 miles per gallon of targets. The largest discrepancy in the past has been 7 city and 6 highway for the Toyota Prius C-subcompact and the Prius hatchback. Ford’s C-Max and Fusions fall short by as much as 10 miles per gallon overall and 12 miles per gallon in the city.

Via CBS News