Returned from Fargo, North Dakota on Sunday.
I drove at two different speeds to compare fuel consumption, while experiencing a 20 mph tailwind.
Car: 2013 Toyota Corolla, automatic.
@75 mph: 36.5 miles per US gallon, 6.4 L/100km (3050 rpm)
@65 mph: 43.1 miles per US gallon, 5.5 L/100km (2650 rpm)
A 16.4% increase.