I recently bought a '92 W250 automatic that is pretty much all stock. It runs great despite the high mileage (295K) and is overall pretty solid. The speedomoter still "works" but it bounces all over the place and isn't very steady. I've also noticed that even though the overdrive still works in the truck it tends to jump in and out a lot on hills and bumpy roads.
Is the overdrive linked in someway to the speedomoter? Would the speedomoter jumping all over the place be linked to the overdrive jumping in and out?

Thanks in advance.