a vehicle's gas mileage normally decreases rapidly at speeds above 55 miles per hour. a car or truck moving at 55 miles an hour can get about 15 percent better fuel economy than the same car going 65 mph. according to the u.s. department of energy (doe), as a rule of thumb, each 5 mph you drive over 60 mph is similar to paying an additional $1.20 per gallon for gas (at $3.10 per gallon).