I just got my new 3.42 gears installed and have gone through the break-in process. I've noticed that my speedometer is off by about 12 mph. Does the 700R4 that came with these cars have a plastic colored gear that goes in the transmission to correctly calibrate the speed for the gear transmission combo?

 

Yes, the speedo gear must be changed out for one meant for that rear end ratio, a gm parts man can give you the list of available speedo gears. Although a bit different than the cable style speedo gears, it works much the same way. One bolt holds the housing to the side of the tranny and the housing pulls straight out. You'll see the gear then. If you use non stock wheel/tire combo, make sure you factor in tire OD if you want the speedo to be accurate.

 

Might have to change the sensor too if the tooth count of the gear is very different.

 

https://www.thirdgen.org/forums/tran...eedometer.html
See Post #42