Speedo Driver Calculation

[Randalla]

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. 

[Duffman]

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

[SteveJ]

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.

[Randalla]

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