Bought a 1990 k5 blazer 'silverado'? as a project It has the usual rot I have seen worse tho I was not told of course that the rear was blown until I went to put it in 2wd to get if off trailer. I see ther are 3-4 diffrent ratios I am trying to find out what one I have so I can pick up a used rear I have the glove box sticker but dont know what I am looking at any help much appreciated.

 

RPO Code GU6 that is listed on your SPI label signifies that the truck originally came with a 3.42 gear ratio in the axles. Provided that the VIN on the SPI label matches your truck and no one has swapped the axles or gears, that should be correct.

 

remove the cover, count the teeth.
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Late model K5s were usually 342s or 373s (all corp 10 bolts after about 1977) as most of the automatic models were equipped with overdrive 700r4 tranny's
Removing that cover and dividing the ring gear by the pinion gear will for sure tell you...plus it's a good chance to inspect and put in some fresh oil