In street rodding the term 'POSI' is usually generic for limited slip. What you want is a limited slip differential for streetable trouble free performance.

A formula for carspeed based upon engine rpm, tire diameter and final drive (rear end ratio) is;

carspeed=0.002975*rpm*OD/FDR where OD is tire diameter in inches and the FDR is the rear end ratio. rpm is engine speed in rev/min. This equation assumes a transmission final drive of 1:1. You can easily put in the overdrive factor as well if you have one.

Regards, Kitz