We're often asked why cars sold in Europe seem to be so much more efficient than those on sale in the U.S.

Typically, it's accompanied by the mention of some supposedly 80-mpg car sold in Europe--while the best U.S.-market vehicles barely crest the 50-mpg mark.

Now, Reuters highlights one of the main reasons for the discrepancy: Not only are European tests unrealistic, but automakers exploit loopholes in the testing, further blurring the lines between rated efficiency and real-world results.

Year after year, European sales figures have shown a bias towards higher average fuel economy and lower fleet-wide CO2 emissions.

In 2013, the average European car emitted just 127 grams of CO2 every kilometer--3 g/km below European Union targets for 2015 greenhouse-gas emissions.

To put some perspective on those figures, the sales-weighted average U.S. fuel economy of new vehicles has just crossed the 25-mpg mark--equivalent to 218 g/km of CO2.

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Some of that difference is down to the sort of cars people buy. Smaller vehicles and diesels are much more prevalent in Europe than they are in the U.S, and both typically lead to more fuel-efficient vehicles than large vehicles with large gasoline engines.

Cars really are more efficient "over the pond," then, but many come with performance (and size) compromises that few U.S. drivers would be prepared to live with.

Classic Volkswagen Beetle Convertible & 2012 Volkswagen Up minicar, Catskill Mountains, NY, May 2012

Classic Volkswagen Beetle Convertible & 2012 Volkswagen Up minicar, Catskill Mountains, NY, May 2012

Discrepancies between European and U.S. numbers extend far further than that, however. Some of it is down to how cars are tested in Europe--the New European Driving Cycle's test procedure is both shorter and slower than the EPA's procedure.

The European "Urban" test, for example, is 13 minutes long--the EPA's city test is 31 minutes long. The "Extra urban" test takes 6 minutes, 40 seconds; the equivalent EPA highway test is 12 minutes, 45 seconds.

Speeds are different too. During that 13-minute urban test, the highest speed attained is just over 30 mph, and maintained for only 12 seconds.

The rest of the test is made up of slow acceleration and deceleration, while around 2.5 minutes is spent stationary--meaning cars equipped with increasingly common start-stop systems use no fuel at all for those minutes.

2013 Volkswagen Golf BlueMotion (European model), 2012 Paris Motor Show

2013 Volkswagen Golf BlueMotion (European model), 2012 Paris Motor Show

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.

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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 2013

EPA gas-mileage label (window sticker), design used starting in model year 2013

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.


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