Speed kills your gas mileage and your wallet. Yes, you've heard it before, but how about some real-world numbers to, ahem, drive the point home? Aerodynamic drag is a minor concern in city driving, but it really kills your gas mileage at speeds over 55 mph. In fact, increasing your speed to 65 increases drag by 36 percent! If you do a lot of highway driving, getting to your destination a few minutes early could cost you an extra $510 a year. Keep it closer to 55 mph and use your cruise control. It will pay off. You know what else will pay off, these 36 car detailing tips the pros don't want you to know.