Hi all,
I believe my rear is a 3.23 posi, best I can tell from the previous paperwork. I have 15 inch American Racing rims installed. My speedometer is way off. Last week I was on the highway going to a car show doing approximately 70 MPH and my speedo read 100 MPH. I think I need to change the speedometer gear to match my rear and rim size to make the speedo read more correctly. Any ideas on the size of gear I need?

 

Here is a speedometer gear calculator. It works good but you will need to know for sure your gear ratio.

http://www.tciauto.com/tc/speedometer-gear-calculator/

 

Thanks allot! I will definitely try it out. That's exactly what I was looking for.

 

Unless you are running on the rims only without tires, the size of the RIM is irrelevant. It's the size of the TIRE that matters...