My Speedo is off after 2.73 to 3.42 Rear swap. Its like when i'm really going 70 it says I'm going 85ish. I called the dealer and they said its the speedogear on the transmission. THe part the guy found at the delar said it was $130, than they said it would be like 3 to 4 hours of labor, which is really way too much, thats more than I payed for the new rear with install which was $300.
My friend that I bought the car of who has a 88 Z28 305TPI that had the same 2.73 rear than went to 3.23 gears said he had his friends shop do the work and it was really cheap, like $30 for the part. SO I called that guy up and he's looking into it and I'm calling him tommarrow. Do you guy's know the part number of what I need?

The car is noticably faster but it hums a lil bit when I drive it and is louder between 35-55mph but lower after, is that normal? I called the mechanic that did the work and he said he didn't hear anything when he drove it, but if I want to bring it in so he can check it out. The weird thing is he said my car a 89 Firebird Formula w/a 305TPI with a 2.73 rearend had a posi on it. That was kinda weird cause I thought it didn't have one stock or am I wrong? But he managed to use that posi on the new 3.42 Rear anyways, I'm thinking maybe it could be the posi that causeing the noise. Please anyhelp I would appreciate.

 

For 3.42s and elec speedo, you will need a
15 tooth gray drive gear p/n 8642620
41 tooth yellow driven gear p/n 25513048

 

Both gears from GM should not cost more that $25.

There is a tech article that explains how to swap speedo gears on an automatic. If you move slowly you can do the swap in an hour, which is about as much labor a mechanic should charge.

 

With the electric speedo, when swapping speedo gears do you need to change to a different vss, or can you reuse the old one?