Slow Down, Save Fuel and Money at the Pump

slow down save fuel U.S.

How much does your fuel economy fall — and how much does your fuel cost rise? — if you drive at 60 m.p.h. rather than 50? How about 70 m.p.h.? 80?

Would you believe a 41% decrease in fuel economy from 50 m.p.h. to 80? That’s like paying $1.38 more per gallon of gasoline, according to the U.S. Department of Energy’s website

Until last week, nobody had quantified the effect real-world speeds have on fuel economy, despite the fact that most people hit those speeds on the highway every day. New research by the DOE –which works with the EPA to generate window-sticker mileage figures — should help create models to help drivers figure out the effect higher speeds have on their specific vehicle.

The differences from one vehicle to another are surprising, according to Brian West, a development engineer who worked on the study at DOE’s Oak Ridge National Laboratory in Tennessee.

The smallest drop in fuel efficiency was 6.9% from 50 to 60 m.p.h., while one vehicle lost a surprising 26% between 70 and 80 m.p.h. Other vehicles saw their fuel economy decrease as much as 18.3% at 60 and as little as 10.8% at 80.

There wasn’t much correlation between decreasing fuel economy and the vehicles’ frontal area and aerodynamic drag. That’s one of the surprises the DOE team is still trying to figure out, because it seems to violate a law of physics.

The study compares 74 vehicles’ fuel economy at 50, 60, 70 and 80 m.p.h. All the vehicles were tested on chassis dynamometers using Society of Automotive Engineers standards. DOE tested 24 of the vehicles in Oak Ridge while Chrysler provided data from 50 vehicles it tested at its labs in Michigan. Vehicles of all body styles, drivetrains, and major manufacturers were represented.

While the figures will vary from one vehicle to the next, West said you can use the average decreases for rule-of-thumb estimates of speed vs. fuel economy. You can assume a 12.4% decrease at 60 vs. 50 m.p.h., 14% at 70 vs. 60 m.p.h. and 15.4% more at 80 m.p.h. Thus, 41.8% lower at 80 vs. 50.

Then you can figure out exactly how much more it will cost you to drive to Chicago at 75 m.p.h. than 50, and whether the money you save at the slower speed is worth nearly two hours longer on the road.

Photo Credit: Shutterstock

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