My 91 300SE revs something like 3300rpm at 65mph. If your overdrive wasn't working or if you were in third gear, you'd be closer to 4500rpm when doing 65mph. In fact, drop the shifter into third while doing 65mph and you'll feel the difference and see it on the tach.
I also notice a pronounced transition state. If I accelerate birskly, the tach will lead the speedo by something like 500rpm. I mean that during brisk acceleration, the tach will read 500rpm higher than it does when cruising at the same speed. I thought my torque converter or tranny was about to quit, but I was assured that the behavior was normal. That's what I get for growing up on 5 speeds and big V8s with no tachs.
I don't think that anything's wrong with your car, but you might see if fresh tranny fluid brings the revs down a couple hundred rpm at 65mph.