Thanks everyone, and a good point you raise Short 3/4, perhaps it is dragging. I think the 'easiest' solution for me might be to pull the gauge again, replace the odometer gear from my 'new' unit into the original gauge...and hope that it isn't the source of the dragging! I'll do it eventually, once my cable nut PTSD has dissipated! If it wasn't for that, the job of pulling the gauge cluster and reinstalling would be pretty easy.
Theastronaut - that's a good idea re checking the odometer reading, so I'll give that a shot. As a complete aside, I see they've finally made a movie using your name..but I doubt it features any pick-ups in it