Nothing ruins a car faster than overheating.
Just because you're nice and cool in the passenger compartment doesn't mean that your engine isn't dying of heat prostration a few feet away. Drive at 55, not 65. Sure, you might be able to drive at 70 miles an hour, but you're doing it at the expense of your engine. You might get to your destination a half-hour earlier, but you might just toast your engine on the way. If you do notice your temperature indicator starting to rise, turn on the heat in your car.