Both the 2013 Ford Fusion hybrid (below) and the 2013 Ford C-Max Hybrid (above) miss their EPA-estimated fuel economy by about 20 percent, according to testing conducted by Consumer Reports magazine. The magazine, which buys all its cars through conventional channels and then conducts real-world fuel economy testing, said the discrepancy is the largest it has seen among current car models. The magazine's data is now being reviewed by the EPA.

In its tests, the magazine found that the Ford Fusion hybrid achieved 35 city/41 highway and 39 combined mpg, while the Ford C-Max hybrid got 35 city/38 highway/37 combined mpg. Both cars are rated at 47 mpg across the board—for city, highway and combined, according to EPA estimates.

In the tests, the magazine buys the cars from local dealers, logs 2,000 "break-in" miles, then installs a precision fuel meter in the fuel line. City fuel economy is measure at the magazine's test track in a standard course, consistently of set speeds, stops and idle times. Highway tests are conducted on a specific stretch of highway, with the cars driven twice in each direction. Tests are conducted by multiple drivers on each car and only in certain weather conditions, with corrections made for air temperature.

By contrast, EPA testing consists of the cars being driven on a dynamometer (essentially a large conveyor belt) to simulate certain driving conditions.

Consumer Reports said that it recognizes the EPA-disclaimer that an individual's fuel economy results may differ from EPA estimates, but said that typically its combined fuel economy estimates are close to the EPA numbers and its highway fuel economy numbers "almost always" meet or exceed EPA estimates.