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have a 36 Chevrolet 1/2 ton pickup mounted on a 1959 GMC
chassis need to use the 59 GMC diff for now. need to know what diff was used in 59 GMC 1/2 ton pickup and ratio..... no tags on diff so I can order the correct parts from USA to replace the crown wheel pinion with a better ratio.

next question using a TH350 tranny and 27.1" tires what should the correct diff ratio be... Chevrolet 327 motor

busy trying to locate a ford 9" diff here in south Africa.

That GMC had a Spicer 45 rear end as opposed to Chevy, from what I've read, and the trucks generally had either 3.08 or 3.92 ratio's - not sure if there were other mid-range ratio's or not, but I think most were one or the other. A quick check is to raise a rear wheel (both if posi, tranny in neutral) and rotate the wheel while counting the revolutions of the drive shaft. I would maybe do ten revolutions of the wheel - almost 31 turns of the driveshaft tells you 3.08, almost 40 tells you 3.92.

Depends how you're going to use the truck on the ideal ratio. If you're highway driving, with 27.1" tall tires and no overdrive you'll be turning right at 2675rpm at 70mph with a 3.08, and right at 3400 with a 3.92. You can ratio from both points if you find you have some other rear gear ratio.