As I just stated in another thread: especially in fall, winter or spring, you have to pay attention what tires are mounted on the car. Winter tires often can only stand a certain speed (usually 160 or 180 kph, depending on the car, i'm not familiar what the car rental companies will use). In this case you will find a little speed-limit sticker on the dashboard. Sports cars might have class H tires (limit: 210 kph) or better. (KLICK ME and scroll down a little)