If I swap my rearend, stock open dif with 3.23's I believe. With another rear with a truetrac and 4.10's how far off will my speedometer be? I will have about 1 1/2 hour drive after the swap before I get home and have an opportunity to fix it. I dont need any speeding tickets, thanks in advance.

 

About 20%. Your speedo will be reading faster than you really are. Just go with the flow of traffic.

 

I went from 33" tires to 31" tires and my speedo is pretty far off...it says I'm doing 85 when I'm maybe touching 70 (similar affect as the gear swap you're planning)...luckily my re-calibrated PCM from PCMforLess was at my front door when I returned home from work today. Probably pop that in tomorrow am