[Ironhorse]

Several years ago I had the same issue with my speedometer being off. 71 4x4 after a swap from manual to auto transmission. I run 31x10.50x15s on 8'' rally wheels. I noted my speedometer reading against a GPS phone app and figured the difference. Contacted www.transmissioncenter.net and gave them the information. They built me an adapter that after an easy 15 minute installation was dead on with the speedometer and the phone app GPS speed. Just attached to the side of the transmission where the speedometer cable attaches.
Think my cost then was around $65 plus shipping but I'm sure costs are higher now. Very easy to do without going in to the transmission and changing the plastic gears.