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Slow down, you move too fast? Even a 10 mph increase in vehicle speed means a sharp reduction in mileage, according to a new study.

Studies show that fuel economy has become the number one concern among American car buyers – but motorists have also been pressing for higher speed limits which can sharply reduce the mileage a car gets.

While that might seem common sense, 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 50 to 60 miles an hour, we find for the largest number of vehicles fuel economy will go down about 12%,” said Brian H. West, a researcher at the Oak Ridge National Laboratory, in Knoxville, Tennessee, where the study was conducted.

Pushing a little faster, you’ll see mileage drop about 14% going from 60 to 70 mph, while fuel economy will dip yet another 16% if you nudge the speedometer up to 80.

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 40 miles per gallon at 50 miles an hour would see something closer to just 25 mpg at 80 mph.

Highway speeds have risen significantly since Congress lifted federal restrictions first passed during the energy crises of the 1970s 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.

The increase in speeds has created havoc for both the EPA, which is mandated by Congress to oversee automotive fuel economy standards and testing, and the automakers depending on EPA results.

The tests that determine the figures on a new vehicle’s window label, the so-called “Munroney Sticker” have been revised a number of times over the past four decades, most recently with the start of the 2012 model-year. The latest procedures include a brief burst at 80 miles per hour but the results generally reflect much less aggressive, low-speed driving, which means that a large percentage of American motorists will get lower fuel economy than indicated.

And the faster they go, the new Oak Ridge study confirms, the faster the gap will grow.

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