junk yard memory tool, divide teeth by 10 to get 'pretty close' rear ratio. 39 tooth is a 3.91, 27 tooth is a 2.76, 32 is a 3.23, etc. Works pretty good unless you have non stock tires. Easy way to find unknown rear in a roller with no jacks (SG or open) is to chalk the driveshaft (long diagonal mark across 1/2 of the shaft) and roll the car forward 1 tire revolution (use valve stem as reference) just count the mark as it rolls. Diagonal so you can pick up the estimated location if its on the other side. Just dont roll over yourself.
