Where can I locate any markings on the rear differential to determine some info? I have a '69 C10 with a 12 bolt rear.
Don't scrape too hard either or the lightly stamped numbers on the axle tube will come off. Use something like a green scrubbing pad - not steel wool unless it's very fine grade. The front axle will usually have a tag hanging on the bottom of one of the bottom axle bolts with the front ratio. The rear gear ratio will not be far off from that - like 3.07 front and 3.8 rear for example. ( My front is a Dana 44 & rear a corporate 12-bolt ) Other than that you have to unbolt the rear housing and count teeth on the ring gear to figure your ratio. This link will help with formulas and ID - http://www.chuckschevytruckpages.com/
Count the teeth of the ring gear and the pinion gear divide the two and that is your gear ratio. Or try this site. www.drivetrain.com/GMtrkratio_posidata.html Steve
Hello there if you do this as Joesgarage has said. If you left the diff off the ground take off the cover turn the pinion around a little at a time. Look at the center ring most GM rings will tell you the # your looking for, you can still count them. Use the math sytem that Joesgarage gave link, to calculate the gear. Here is a pic showing the # stamped not the best pic but if you can zoom in you should see them http://img.photobucket.com/albums/v466/Pontiac1976/Pontiac1976 truck parts/75DiffgearIMG_00.jpg
If all you want to know is the gear ratio, you don't have to disassemble anything. Jack one wheel off the ground, put the transmission in neutral and the parking brake off. Now try to turn the wheel. If the wheel turns easily, turn it exactly TWO revolutions and count how many times the driveshaft turns. That number (including any fractional turns) is your axle ratio. If the wheel is difficult or impossible to turn, this means you may have posi. In that case, jack both wheels off the ground and turn either one exactly ONE revolution. Count the driveshaft turns as above. Ray
