My 77 300D is a great car. I've owned it for over a year and put about 15k on, which puts me just below 200k. Runs great and looks good. I have been curious about the speed these cars were meant to go. In 1977 there were no doubt freeway capable autos, but I think I remember reading in the manual that the top speed is recommended no higher than 83 MPH. I have gone just about this fast in the car and the engine was loud.

My question is, what is the difference between driving 55 and 65? 55 seems optimum as far as fuel efficiency but most drivers don't tolerate someone going that speed. At 65, although louder, i am able to accelerate. Is the engine going to wear more at higher RPM's? As I have no tachometer i can only base my assumptions on the sound of the engine. What might the RPM be at 55, 65, 75 as a percentage of maximum and are there negative effects of travelling at higher speeds?

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