I had a bulliten earlier about what speedo gear i need for my car with a 4.10. I have a 91 ford mustang lx with a 4.10 rear end and a shift kit. I hear that the 23 tooth is the gear i needed so i ordered the one from 50 resto and put it in but i didn't seem to work. So im not sure if its just the gear or what the problem could be but if i need a different gear or there could be somethin else wrong could u guys let me know what u think. Thanks

Trent

 

The problem isn't that the mph is off its that the needle just bounces around. i dunno if thats caused by the gear or what causes that.

 

your cable is probably bad, or not completely mating with the gauge itself... you might have to pull the gauge out and lube the cable inside

 

If the cheap fixes don't fix the problem (speedometer gear and cable), then it is most likely the speedometer itself.

The 23 tooth is not a Ford part, it's a close fit to the Ford gears, but will typically get chewed up from it's size difference. If your gear is ok and the cable still moves freely (and not fried by an exhaust pipe), check the speedometer. I tried the gear and cable swapand finally pulledthe speedometerwhen the needle was bouncing. Apparently the rear bracket has a tendancy to bend downward (as told to me by the repair shop). After rebuilding the speedometer, the needle was spot on once again.

FYI: Rebuilding the back of your stock speedometer can be done without effecting the odometer reading.

As for the correct speedometer gear.... I assume by your post you have a 91 with an automatic. The 23 tooth gear will be close but not accurate. Here are a few charts to get you in the right direction.

http://mustangpartstech.com/SpeedometerGears.html