While each vehicle reaches its optimal fuel economy at a different speed (or range of speeds), gas mileage usually decreases rapidly speeds above 55 miles per hour. Just slowing down from 65 mph to 55 mph can increase your miles per gallon by as much as 15 percent. According to the U.S. Department of Energy (DOE), as a rule of thumb, you can assume that each 5 mph you drive over 60 mph is like paying an additional $0.21 per gallon for gas (at $3.00 per gallon). So, the Fuel Economy Benefit of driving slower is between 7 and 23 percent. The equivalent gasoline savings is $0.21 to $0.71 per gallon. Let's take two hypothetical drivers, Bill and Bob.