I'm afraid I don't know where it is on the post 1972 players, but I do work on the old t400 8 tracks and there is a speed adjustment on those so I'm sure there is a speed adjustment on the newer models. There is usually a hole in the cover that you can see a small screw head through. One the newer ones, you may have two holes like this. One being the antenna trimmer for AM and the other being the 8 track motor speed. The antenna trimmer is usually on the fron next to the tuning knob. As I said, I don't know where the adjustment is on the newer model 8 tracks but I kinda doubt you'll be able to reach it easily while the radio is installed in the car.
I can't imagine that the belt or the tape itself being loose would cause the speed to be too fast. The motor turns the capstan at a set speed. The capstan is what drives the tape itself. So if the tape wasn't pushed up against the capstan well enough, the tape would actually drag/slip. Not run faster.