My 52 GMC 3/4 ton is equipped with a late 60s GM 10 bolt rear end. I'd like to figure out the ratio without opening up the cover. My buddy Ken and I did the "chalk mark" measurement today. One turn of the tire resulted in an estimated 3 and 5/8 turns of the driveshaft or a ratio of about 3.62:1. The down side is that I don't think GM made such a ratio. I know GM had the following ratios, 2.41:1, 2.56:1, 2.73:1, 3.08:1, 3.43:1, 3.55:1, 3.73:1 and 4.10:1. A 3.63:1 would be mathematically easy using an 11 tooth pinion and a 40 tooth ring, but I have not found such a beast (yet) using Google. Any of you have first hand knowledge of such a ratio, or a pointer to a web page showing one?
With a standard open (non posi traction) differential raise one wheel off of the ground, mark the tire at the 6:00 position, mark the drive shaft. Turn the tire exactly 2 full turns. Count the number of times that the drive shaft turns. 2-3/4 turns of the drive shaft = 2.73, just past 3 turns = 3.08, 3-1/3 = 3.23 or 3.31, almost 3-1/2 = 3.42, just past 3-1/2 = 3.55, 3-3/4 = 3.73, just past 4 = 4.10 or 4.11. With a good solid limited slip differential (posi) raise both wheels off of the ground and turn both wheels exactly 1 full turn. Everything else is the same as above. You can also do this with a standard differential as long as both wheels turn exactly 1 full turn.
No OD on my truck, but 31" tall tires makes for around 2300 RPM @ 60 MPH or 12.5 MPH at full scream in granny gear.
