I want to get the speedo to read correctly. I have installed a 3.73 in my 91 5.7 Z-28 and it's a little off GM parts guys say they can only tell me up to 3.42.Any help would be great

 

With 245/50/16 tires, you need a 15T gray drive gear and 45T blue driven gear.

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1989 IROC-Z 5.7L
NOS 150HP kit
ProBuilt 700R4, PI Vigilante 2800 stall lockup
Baer Brakes 12" Sport System

 

I did this swap on my 91, here's the part numbers. I had 2.73 originally, so I had to change the drive gear along with the driven gear. You probably already have the correct drive gear (be grateful). I'm not sure which number is the driven gear (the one you need) but if you give the dealership both numbers they should be able to tell you. My speedo reads about 1 mph too high at 50.
#8642620 gear
#25513051 gear

I also replaced the sensor with this part number. I'm not sure if it's necessary, but the guy on the board who gave me the gear numbers gave me this number too. Since it was different from the part number on the sensor I already had, I changed it. You may not need it though.
#10456089 sensor

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91 Z28
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