I have a 1991 LX 5.0 w/AOD transmission and I installed a 4.10 rear end, so I purchased this gear along with a new speedo cable (I had the jumpy speedo problem) and while it (mostly) cured the jumping problem, the speedo is still off by 10mph. Why? Also anyone know why the speedo would still be a little jumpy? It's only jumping 2-4mph it's still happy.
Because you have a 91 with an AOD, the internal drive gear in the transmission is an 8 tooth unit. Even with a 23 tooth driven gear, you will be 10mph off with 4.10 gears. Without a custom reduction box form someone like Abbott, this will be as close as you'll get.
Jumpy speedometers are typically an old/worn cable, a new cable that was not properly lubricated, or a defective speedometer itself.
Your magnet in the speedo itself could be broken. For speedo repairs contact specmo.com One warning if you remove the cluster, don't drive the car! The charging system won't work because the removal of the volt meter breaks the circuit from the alternator to the battery.