Speedo in op on 4 speed OD trans.

So I take it that for any given pinion gear there is only one location where the gear will mesh with the trans shaft and that is the ratio you have. I have P225/60R 15 tires and they are approx. 25.5" in Dia. and the rear end is set up with 355 gears. What speedo gear pinion do you think I need?