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A few recent, high-profile cases have brought fuel economy testing procedure into question.
When Kia and Hyundai are being sued for inaccuracies in reported EPA figures and Ford hybrid drivers are missing out on official numbers by 20 percent, suggesting the designs of some vehicles may enable them to pass tests without that efficiency making it all the way to the road.
Still, it could be worse--you could be European, where some carmakers are really cheating the tests.
The Guardian (via Autoblog Green), says that some carmakers are over-inflating tires, taping up gaps and adjusting brakes for less resistance to boost their official figures. This results in misleading fuel figures for the customer, and artificially bumps some cars into lower CO2-based tax brackets, a major factor for European car sales.
While there are some impressively efficient cars sold in Europe--and significantly more to choose from than in the U.S.--the discrepancy between quoted and real-world figures can often be huge.
In the UK, drivers are achieving 87 percent of the official figures on average--not too bad, considering we usually suggest a 15-20 percent discrepancy between EU and EPA numbers, car-for-car.
But some are as low as 70 percent or less, often at the most economical end of the scale--small-capacity turbocharged units, for example. These are cars essentially designed to ace the tests thanks to optimized gearing or fuel mapping--but they don't work so well in normal driving conditions.
The additional cheats being used in testing can put those figures even further out of step--as most drivers don't pump up their tires, or tape up panel gaps to boost economy.
Irritatingly for many, the carmakers aren't even breaking any rules. According to Transport And Environment, who carried out the research, "There is no evidence that carmakers are breaking any formal rules--but they don't need to--the current test procedures are so lax there is ample opportunity to massage the test results."
Even prior to some of these cheats being discovered, there have been several calls for the testing procedure to be altered to give more realistic real-world numbers.
The current test includes an "urban" figure derived from only 2.5 miles of driving after a cold start; an "extra-urban" number over 4.3 miles of higher speeds--a maximum of 75 mph, an average of 39 mph--and an "average" of the other two, weighted by distance. Gear-changes are made at predetermined points (in manual vehicles), and all tests have the same starting, stopping and idling times.
A full exposé on the procedure can be found in Transport And Environment's pdf document.