You may need to get a speedo cable calibration unit.
It is like a gearbox for your speedo cable.
Later AODs and 4.10s don't really work togther when it comes to the speedo gear.
There is a 23 tooth unit that gets close, but the teeth are so thin that the gear disintegrates in a relatively short time.
My son's lasted almost a year...
The stock range of speedo gears work correctly with a limited range of tire sizes. The 23 tooth white gear works good for some guys, but self destructs in a few months for others.
Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy within 1%-3%. The drawback is that it isn't cheap.