i have searched but dont really know how to figure which speedo gear i need. i just bought a 99 z28 rear end with 3.42 gears, i currently have the stock 10 bolt with 2.73 gears. i am running 17/40/285 tires on the back. i guess i really just need the gm part # for the right one.

Check this thread:

https://www.thirdgen.org/forums/tran...eedometer.html