Speedo cog from 200SX (long)

Hi all, I have an interesting question for the experts on this site especially jmortenson. I found an 86 200SX 4 cyl N/A at the JY and removed the KR180 diify. Upon removing the rear cover I found the numbers 37.9 stamped on the ring gear. This indicates that it is a 4:11 which is what I want to put in my car, a '73 with an 81-83 5-spd. I will take the buttons from the spider gears from my R180 3:36 and put them into the 4:11 thereby utilizing my half-shafts and stubs. Thank you, jmortenson for the diffy sticky on hybridz.

Now here is the intersting part. I also removed the speedo cog (which was in an aluminum housing) from the 5-spd in the 200SX and here is what I found: It has 21 teeth and is tan/gray in color. On Dave Little's Datsun Z Garage he states that the 4:11 gear is red, somewhat larger, and has 20 teeth and the 4:39 gear is purple and has 21 teeth. I also removed a speedo cog (electric) from a 240SX and it was red, 20 teeth, and larger than the cogs I've been dealing with.

So does anyone know what I have here, and will my speedometer reading be way off if I use it? Could I use the red cog even though it's larger? It seems to just slip into the tranny with no problem. The 200SX appeared to be a standard plain old car with no modifications that I could spot. I hope this is understandable. Any and all advice will be welcomed.

Cheers all, Mike