I just swapped in a 145 MPH mechanical speedometer into my 1984 Z28. I have a rebuilt T-5 W/C tranny. The speedometer reads about 20 mph too high now. I'm running 3.73 rear gears with stock tires. Will I have to replace the Drive and Driven gears? What gears would I need? I searched for a tech on how to replace those for a manual transmission but couldn't find anything, and last but not least, someone once mentioned to me about a "reduction box." Is that something that attaches to my speedo cable where I can just "dial" in the calibration?

 

It doesn't matter that the replacement trans is a "WC".

Sounds like whoever put in your transmission, just didn't bother to put in the right speedo gears (since your car is a L69...). Or, you changed your rear gear, and didn't make the corresponding adjustment to the speedo gears.

Take out the speedo gear "sleeve", and look at what's in there. You should have a 7-tooth blue/purple drive gear on the mainshaft, and 22-tooth red driven gear. Your odds are good that you have the right drive gear, you probably have something like a white 19-tooth white driven gear instead though.

If your "sleeve" is chinesium, replace it. Those OE POSs wear real bad and allow the driven gear to move away from the drive gear, and the teeth don't mesh properly, and the gears tear up. The new ones you get at the dealer are steel, and will make your speedo work much better over the long run, by preventing tooth misalignment.