Are you an aggressive driver? If so, you might be wasting tons of gas and money. When you drive at speeds at or below 55 miles per hour, your car is working at maximum fuel efficiency. Go any faster, and for every 5 miles in speed above 55, you decrease your fuel efficiency by 10 percent. So if you're sailing at 75 miles per hour, you've wasted 40 percent fuel efficiency on your car. Stay safe, save fuel and drive at reasonable speeds whenever possible.