rear end gearing question

[chuckf]

I posted a question a while back on how to determine the rear axle gearing in my 97 2500 suburban. I jacked up the rear end. Marked the tire and drive shaft. Counted the number of times the drive shaft made a complete revolution for each tire revolution. I rotated the tire 10 times, the drive shaft rotated 20-1/2 times. So this tells me the gear ratio is ~2:1. This makes no sense to me. I was anticipating the ratio to be in the range of 3.7:1 to 4.1:1. Am I doing calculatin wrong? Here's another data point to consider. The reason I had the rear end jacked up was to replace the brake pads and have the drums turned. When I went to get the pads from the local parts store, the pads that matched up were for a 1-ton dually, not a 3/4 ton single. Here's some more info to consider. I bought the truck at an auction, the back bumper had some minimal damage. Could a 1-ton axle fit on a 3/4ton suburban? This would explain why the brake pads were different. The pads that fit are 3-4" wide. Any help on determining the gear ratio and brake shoe size difference would be appreciated. Thanks. pcfaisst@netscape.net

[cruzer]

You probably have a 14 bolt full floating rear end which was put in some 3/4 tons and most/all
1 tons. You should turn the drive shaft not the tire. Count the # of revolutions the drive shaft makes for 1 tire revolution. You can look in the member section for the RPO code list and get the codes for the gear ratios , then you can look in your glove compartment for the corresponding code. This works if the rearend has not been changed. You can always pull the cover and change the fluid and caculate the #'s on the ring gear

[Old6.2]

jackf, By your description I have to assume that you only jacked up one side of the rear and the differential came into play. The diff will split speed equaly in a turn. If the inside tire slows to 90%, the outside tire will increase to 110% of input speed. With one tire on the ground (0%speed)the outer will be at 200%. This will require you to multiply your 20 1/2 turns by 2 (41 turns) and divided by the 10 rotations of the drive shaft which would point to 4.10s. As cruzer said, the best way is to pull the cover, check the pinion/ring gear teeth and switch to synthec gear oil!

[Old6.2]

chuckf, excuse me for posting your name incorrectly!!

[MikeC]

chuckf,

I am happy to report to you that the axle is probably what came in the truck. The diesel Burbans have 3/4 ton running gear in the 1500 series and one ton running gear in the 2500's. Your axle should be a 14 bolt full floater. Your brake shoes should be 3 1/2 inches wide by (IIRC) 13 inches in length, and yes :D , they are for the duallies also. According to a mechanic at work who is a D****/Cummins fan, "These are the biggest f'ing drums I've ever seen, wish my brakes were this heavy."

Mike

Oh yea, the ratios offered to the best info I could find are either 3.73 or 4.11 The sticker in the glove box will give you a code. The code & year or just the VIN should give a dealer enough info to tell you the ratio. I used to remember them off the top of my head but I can't find it now, old age.

mc

[chuckf]

Thanks for everyones help,
I did have both wheels jacked up off the ground when I rotated the tire. I will check the RPO codes and VIN number. I think I will also pull the cover and check the ring gears to know for sure. I will also change to synthetic gear oil.
Thanks again. smile.gif

[CareyWeber]

chuckf,

If you don't want to pull the cover look at the pinion end (where the nut is holding the yoke on) the number of teeth on the pinion and the ring gear are stamped there divide them to get the ratio.

Carey

[catmandoo]

jack up 1 side and rotate tire 2 times count shaft revolutions unless its also yes the numbers are usually stamped on one of the axle tubes may take so wirebrushing to find but they are there.