From NBC News…
“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 percent,” said Brian H. West, a researcher at the Oak Ridge National Laboratory, in Knoxville, Tenn., where the study was conducted.
You’ll lose another 14 percent pushing it t0 70 mph, and another 16 percent on top of that doing 80. The researchers found this was true in all vehicle classes, not just clunky SUVs or trucks. So which do you want: extra minutes or extra dollars?