Compared to modern, more powerful cars equipped with an overdrive top gear, your 250S is going to seem a bit revvy at highway speeds. In order to get any kind of takeoff performance from a 2.5 liter engine in such a heavy car, overall gearing is lower than in most new cars.
I assume that your car is sound (clutch is not slipping).
Try this link re: 250S performance parameters. When healthy, this is a 180kph/110mph car.