Increase Speed, Decrease Mileage No Matter What You Drive

A new study clearly quantifies the impact of putting the pedal to the metal. And it finds that there are surprisingly few differences between vehicles, whether brick-like SUVs or sleek, wind-cheating sports cars.

"People really like rules of thumb and if you’re increasing your speed from fifty to sixty miles an hour, we find for the largest number of vehicles fuel economy will go down about twelve percent," said Brian H. West, a researcher at the Oak Ridge National Laboratory, in Knoxville, TN, where the study was conducted.

Pushing a little faster, you’ll see mileage drop about 14 percent going from sixty to seventy mph, while fuel economy will dip yet another 16 percent if you nudge the speedometer up to eighty.

While West says he "wasn’t surprised" by the general results of the new study, he and the rest of the research team did make an unexpected discovery. They had expected to see some types of vehicles do better than others, but the reduction in fuel economy was consistent across all vehicle classes, even the heaviest and boxiest SUVs, with only a one or two percent variations from the least to most efficient models.

That included hybrid vehicles which are really designed for stop-and-go city driving where their batteries are constantly being recharged. At freeway speeds, there is little opportunity for them to regenerate energy that can be used to offset the gasoline they consumer. And, in fact, many hybrids actually have lower highway mileage ratings than what the EPA estimates they get in city driving.

While West declined to discuss the political ramifications, the new study could be used by proponents of increased fuel economy – and opponents of higher speed limits. A vehicle that might get forty miles per gallon at fifty miles an hour would see something closer to just 25 mpg at eighty mph.

Highway speeds have risen significantly since Congress lifted federal restrictions first passed during the energy crises of the 1970's that, at one point lowered the maximum U.S. speed to 50 mph and then raised it to 55, the infamous "double-nickel."

Late last year, Texas opened the country’s fastest highway, a toll road stretch near Austin with a top speed of 85 mph.

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