Even Hybrids Get a Lot Less Fuel-Efficient at Highway Speeds

Posted: September 14, 2009

To reduce your carbon footprint, start with your right foot.  Ease up on it a bit.

Consumer Reports recently tested exactly how fuel-efficient some of America's highest-mpg cars are when drivers push them up to what, in most places, qualifies as normal highway speed. 

CR explains, "The results show the faster you drive, the more fuel you burn-no surprise there. But the most significant change in fuel economy comes from the most fuel-efficient vehicles we tested."  The magazine put seven vehicles to the test, measuring fuel-economy at 55, 65 and 75 mph.  "The Honda Insight hybrid showed the largest drop in fuel economy," with a drop of 7 mpg travelling at 65 mph instead of 55 - and a drop of more than 15 mpg at 75. 

But the phenomenon isn't limited to hybrids.  CR reports, The "Toyota Camry returned 40.3 mpg at 55 mph, but that reduces to almost 35 mpg when the speed moves up to 65 mph and drops to almost 30 mpg when speeds reach 75 mph."  The vehicle least affected by higher speeds, ironically, was the least-efficient tested.  The Mercury Mountaineer SUV saw a drop of only 6 mpg with the full 20 mph speed boost.

Kicking Tires comments, "The EPA fuel ratings are a useful tool for generally judging what kind of mileage one can get out of a car, but real-world driving often changes the formula due to a number of variables."  In some places, the EPA ratings may not be particularly meaningful, since state speed limits can vary from the 55 the EPA test assumes.  In fact, many buyers complain that their cars don't match the fuel-economy promises by the EPA numbers - and the reason may simply be speed.

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