Each person will drive his or her car differently and in different traffic and weather conditions, all of which will affect fuel economy.

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ord is the latest automaker—joining Hyundai and Kia—to face a lawsuit over fuel-economy claims. The automaker has claimed Environmental Protection Agency (EPA) city, highway, and combined fuel-economy numbers of 47 mpg for both its C-Max Hybrid SUV and its Ford Fusion Hybrid midsize sedan. Owners are reporting numbers that are lower (in the high 30 mpg range) and Consumer Reports got similar numbers during its testing.

To us, what's interesting isn't that the numbers are off, but what they mean in the first place. In most cases, the manufacturers perform their own in-house testing, with the EPA signing off on those numbers (The EPA verifies the testing on 10 to 15 percent—about 150 to 200—new models each year). For their part, the manufacturers use pre-production prototypes and a test that is standardized by the federal government.

2013 Ford Fusion 2013 Ford Fusion

This means that the automakers are more or less bound by the honor system, at least in a way. But even the EPA's test isn't perfect. It's performed in five cycles (city, highway, high speed, air conditioner on, and cold weather) on a dynamometer (a device in which a car is mounted in order to measure data) in a lab. Even though speeds and the number of stops are specified, that doesn't mean the test can match real-world conditions precisely.

Therein lies the problem. Each person will drive his or her car differently and in different traffic and weather conditions, all of which will affect fuel economy in different ways. This makes it difficult to prove if an automaker intentionally lied about results or otherwise fudged the numbers, or if a driver is missing the mark due to his or her habits.

It's true that in the Ford case, and in the case of at least one Kia model (the Soul, which was off by six mpg), the numbers are off by a lot. Whether through misleading data manipulation or honest mistakes made while testing, Ford came up with the 47 mpg figure, and once the government signed off, Ford touted that number since higher fuel-economy numbers mean a lot to consumers in these days of high gas prices.

What this means for consumers is that until a test is devised that can better reflect real-world conditions (and given the variety of driving styles and conditions, this may well be nigh impossible), the EPA's numbers should be taken with somewhat of a grain of salt. Buyers should look at car reviews that perform real-world fuel-economy testing along with quizzing current owners of a car they have their eye on (perhaps using internet forums) about their real-world results. It also helps for drivers to be aware of their own habits and commuting conditions.