The 2013 Ford Fusion and C-Max hybrids are two of the best cars on the market when it comes to fuel economy, according to the Environmental Protection Agency. However, magazine road testers, and customers, are having trouble achieving the same lofty mpg numbers in real world circumstances.

Both the Fusion sedan and C-Max hatchback are powered by a 2.0-liter, four-cylinder gasoline engine and an electric motor with lithium-ion battery pack. The powertrain produces a combined 188 horsepower in both cars and is rated at 47 mpg in all three EPA categories (city, highway, combined) in both cars.

Green Car Reports tested the C-Max twice, once at a Ford media event, and once unescorted. The magazine reports that, the first time, the C-Max could only muster 37 mpg over 50 miles of mixed freeway and urban driving. The second time around, testers wrung out 40 mpg over 240 miles of mostly highway driving.

Consumer Reports is saying the same thing. It tested a C-Max and averaged between 33 mpg and 39 mpg, as registered on the car’s trip computer. While the magazine has only had its Fusion Hybrid for two weeks, it says the average so far is 40 mpg.

Fueleconomy.gov, the EPA’s fuel economy reference site, invites owners to submit their real world mileage. The average for the C-Max is currently 39.5 mpg (an average of 23 vehicles) and it’s the same for the Fusion (based on nine vehicles). In comparison, the Toyota Prius, which is rated at 51 mpg city, 48 mpg highway, and 50 mpg combined, scored 51.1. The Honda Civic hybrid, rated at 44 mpg in all three categories, scored 48.3 mpg.

2013 Ford Fusion Hybrid front three-quarter view blueWhy the discrepancy? EPA tests are designed to be as realistic as possible, but scientists will never be able to replicate the chaotic world of real-world driving under controlled conditions. For this reason, the EPA does include a “your mileage may vary”-type disclaimer on its web site.

So far, that seems to be Ford’s explanation. Ford electrified vehicles communications manager Wes Sherwood told Automobile that fuel economy is significantly affected by the way a car is driven.

“No matter how you want to drive, the car is capable of doing it,” Sherwood said. “If you’re out to maximize fuel economy, we have the technology to help. If you want to drive [the C-Max Hybrid] like a normal car, the vehicle has the performance to help.”

That was Car and Driver’s experience as well. Testing the C-Max Hybrid, the magazine averaged 32 mpg, concluding in its December 2012 issue that, “driving it like a real car yields real car fuel economy.”

It was the same story with the Fusion, which also averaged 32 mpg. However, Car and Driver noted that the Toyota Camry Hybrid and Hyundai Sonata Hybrid averaged 30 mpg and 27 mpg, respectively, in its tests.

Real world conditions and driving style may play a big part in a car’s actual fuel economy, but Ford could be faulted for stretching the truth if its cars’ mpg numbers are completely unachievable in real life.

Hyundai and Kia found this out recently. The Korean brands had to compensate owners because many of their cars’ fuel economy numbers were exaggerated, sometimes as much as six mpg. The discrepancy was reportedly due to an error in EPA testing.