2013 Volkswagen Golf BlueMotion (European model), 2012 Paris Motor ShowEnlarge Photo
The EPA's city test reaches almost 60 mph on occasion, while the rest of the drive is spent accelerating to 30 mph and stopping again--true stop-start driving, and much harder on economy.
It's the same with highway testing--not only are the EPA's tests longer, but cars spend much more time at greater speeds. (Though the basic CAFE highway test cycle still only tops out at 60 mph, and averages a mere 48 mph, so it's hardly realistic either.)
The EPA also considers extra variability, such as 'High Speed', 'Air Conditioning' and 'Cold Temperature' tests, to adjust the city and highway mileage posted on every new car's window sticker to get it closer to real-world factors.
But as in the States, automakers in Europe have exploited loopholes to game the tests. Reuters cites European Environment Agency data suggesting manufacturers use tests that replicate an unrealistically smooth road surface, and use tires that have extra traction.
There are other, less-obvious factors too. The structured nature of the tests means automakers can set up gear ratios to suit--particularly on automatic transmissions--while gentle throttle-mapping software, ineffective at higher speeds, can lead to better official mileage.
Finally, there's one significant difference some people are unaware of: The often-quoted European figures are measured in liters of fuel used per 100 kilometers covered, the inverse of the North American gas mileage (which is distance per fuel quantity, the other way around).
So when those figures are converted to the more familiar MPG measure--generally for a U.K. audience--they are generally given in the Imperial gallons used there. And those aren't the same as U.S. gallons: They're 20 percent larger, containing 5 quarts rather than the 4 quarts in a U.S. gallon.
EPA gas-mileage label (window sticker), design used starting in model year 2013Enlarge Photo
In other words, that "80-mpg" European car is doing nearer 67 mpg--and it doesn't achieve anything near that in real life, because testing procedures are so far from reality.
Typically, there's a 20 to 25 percent difference between European NEDC and U.S. EPA fuel economy figures: A new Toyota Prius is rated at 60 mpg in Europe, exactly 20 percent more than its EPA combined rating of 50 mpg.
Some of the differences are even higher, though: The 48 mpg of a 2014 VW Beetle TDI in Europe is fully 50 percent higher than its EPA combined rating of 32 mpg.
In real-world driving, the figure is somewhere between the two, and EPA figures aren't always accurate either, particularly on diesel vehicles.
But generally, EPA numbers are much closer to the mark than European figures.
So next time you see a car advertised in Europe with some spectacular MPG figure, take it with a pinch of salt--because drivers aren't getting anything like that in the real world.