i have an 89t auto which i think comes with a 3.73 rear differential. I heard that the N/A has a 4.30. is it possible to just switch them without having to mod it at all? how would i correct the speedometer?
Differential switch
- Thread starter jamesdem1123
- Start date
Then you have to simply deal with it. The speedometer drive gear is unaccessible from the outside of the transmission for whatever reason, hence, you must pull it all apart.
You could buy a ratio box from a speedo shop and put it between the cable halves 
(This is how I got it accurate with the Tremec TKO)
(This is how I got it accurate with the Tremec TKO)
Clip - lower top speed, but you sure get there faster. Well, if you can keep the tires from spinning, anyhow. Makes for a nice drag race machine, if that's what floats your boat.
Both the N/A and turbo auto trans have the same drive gear internally so if you go with the N/A 4:30 rear, just get the driven gear from a N/A auto trans.
They are interchangable and is external (so to speek), its very simple to do. Just unhook the speedo cable down at the tranny and remove the driven gear assembly that the cable was attached to from the tranny (one bolt and it pulls out). Get the unit out of a N/A auto trans car from a auto salavage yard and put it in yours, hook up the cable and off you go with a correct speedo.
They are interchangable and is external (so to speek), its very simple to do. Just unhook the speedo cable down at the tranny and remove the driven gear assembly that the cable was attached to from the tranny (one bolt and it pulls out). Get the unit out of a N/A auto trans car from a auto salavage yard and put it in yours, hook up the cable and off you go with a correct speedo.
Jeff Lange said:
Yes they are different part numbers, but they are both 6tooth worms and the driven is 18 tooth for the turbo and 21tooth for the N/A.
I put a 3.727 in a N/A car about 2months ago for a guy and just got the driven gear from a turbo car (33403-69195). It fit and works fine and the speedo is pretty accurate according to a local sheriff's officer.
Now whether or not they are meshing correctly, I dont know, but everything seems to be working fine. Knowing that they are different numbers, what the difference is, Im not sure, and I may have the guy bring the car back and I'll pull the driven gear out and see if it is wearing funny or anything.
There is a box you can get. my car has it on it. Also, the gear is accessable. there is one bolt where the speedo goes in. you just have to find the right gear.