Here's a sure way. Jack up 1 side of the rear end, unless it's limited slip then both now with vehicle out of gear make a mark on tire at straight up. Now mark your defferintial behind the drive shaft is good or any where you can observe the results. Now turn tire 1 revoloution and have someone count hte revs of the drive shaft or deferintial. Most likely on that truck it will be 3 and a half turns or a 3.50 gear works every time especially if your hip on your axle ratio availability for the specific vehicle in question. Your fixed have fun! Dc