I have a 1972 240Z that I  have recently installed a 390 rear end in. The wide ratio 5-speed transmission currently in the car has a white 19 tooth speedo driver in it which causes the speedometer to read about 5 mph too fast at 80 mph. I'm planning to swap in a red 20 tooth speedo driver and am curious if any of you math whizs can give me an idea of how much closer I'll get to the speedometer reading accurately. I realize there are other variables that come into play here, like wheel and tire size, but I'm just trying to get an idea of what to expect once I get my new close ratio 5-speed installed with the red gear. Thoughts? I'm currently running 16" Panasports with 205/50/16 tires. 

I read somewhere on this site that each tooth, addition or subtraction, equates to a 5% change in mph reading, all things being equal.  

In theory is should be the #teeth old pinion/#teeth new pinion.

Since you're going to a 20 tooth pinion gear, it should register 95% of what the 19 tooth pinion gear did. That would mean it would register about 81 MPH if the 19 tooth gear was registering 85.

Thanks guys. I'll post up here after getting the new transmission and speedo driver installed and report findings.