Higher Speed Limits Offsetting Benefits of More Fuel Efficient Vehicles

A study by the U.S. Department of Energy’s Oak Ridge National Laboratory has found that although the demand for more fuel efficient vehicles has increased, the actual fuel savings for drivers is being mitigated by their driving habits.

During the energy crises of the 1970s, the federal government restricted the maximum U.S. speed limit to 55 miles per hour.  In recent years, however, speed limits on highways nationwide have risen dramatically.  The nation’s highest speed limit is currently 85 miles per hour, on a stretch of toll road near Austin, Texas.

A number of studies have shown that as driving speeds increase, fuel efficiency decreases but there has been a common misconception that advances in automotive technology

Researcher Brian West said, “If you’re increasing your speed from 50 to 60 miles an hour, we find for the largest number of vehicles fuel economy will go down about 12 percent.”  When driving speeds are increased from 60 mph to 70 mph, the fuel economy of virtually all vehicles drops an additional 14 percent.  At 80 mph, fuel economy dips by another 16 percent.

West said he and other researchers had expected some types of vehicles to perform better than others in the test.  Instead, they found only a one percent to two percent difference between the least and most fuel efficient models they tested.

Hybrid vehicles fare no better than conventional combustion engine powered vehicles when it comes to highway driving. When driven at lower speeds, in stop-and-go conditions, gasoline consumption is offset by the vehicles’ electric motors.  Many hybrid models are actually far less fuel efficient when driven at highway speeds than their EPA estimates for city driving.

Last December, Consumer Reports challenged Ford Motor Company’s 47 mpg fuel economy estimates for two of its hybrid vehicles.  In independent tests of the 2013 Fusion Hybrid and 2013 C-Max Hybrid, the magazine reported actual fuel economy numbers of only 39 mpg and 37 mpg respectively.

In an email statement defending the automaker’s claims, Ford spokesman Wes Sherwood said, “Driving styles, driving conditions and other factors can cause mileage to vary.” He also said that some Fusion Hybrid and C-Max Hybrid owners had reported getting better than 47 mpg.

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