Hey soundmark (btw, we use our real names on this forum),
Welcome to the forum.
Remember that it's the pinch roller>tape>capstan interface that controls the speed that the tape moves across the heads (at least on machines that have capstans and pinch rollers). So no matter what you do before and after this interface can only refine or degrade this interface but can't change the speed. So, when you change speeds on your machine, the rotational speed of the capstan changes.