Can't figure out what the ratio is in the 12-bolt on my '65 Suburban. I assume it's the stock one, with stock gears. I've scraped layers and layers of gunk, grime, and dirt off of it, can't find a stamping/code on it anywhere on it. I'm looking on passenger side, top of the axle tube, in between the coil spring and differential. Am I missing something here? My next best bet to jack the whole thing up and count tire rotations to drive shaft rotations? I feel like I'm losing my mind on such a simple task.
It will be on the passenger side, but it may not be on top of the axle tube. Mine was over toward the center, further down on the lower part of the axle tube. You'll find it if you keep looking and scraping.
If I were to slide under the truck, the code would be on the passenger side axle tube and more on the bottom side of that tube...over near the ring & pinion carrier. I've seen them all over the place. Some closer to the wheel and some closer to the pumpkin and rotated any which way. I suspect the axle tubes were stamped and then later installed without a lot of regard to where the code was (and mine looks like it could have been done with a hammer and individual character punch pieces. If you look at the image I attached, it is right in the center of the yellow oval...where the pointer is.
Take off the diff cover. The ratio is stamped on the ring gear. It's the only way to be sure. The gears may be different than the tube code. The tire turn is not accurate. Changing fluid in the diff is not a bad thing. Should be 3.73, as said. 3.73 is the no.1 best all around ratio for acceleration and cruise combo.
Watch out for careful drivers!!! I'm away on an ego trip. Will be back on Feb 30. I'm not an Auto Mechanic, but I play one on TV. I charge $0.02 for every opinion and I take Paypal. Plan B is always better than plan A, by definition. "We have no time for comment, every man will make his own. LET IT BE MADE WITH POWDER AND BALL!!!" I recommend invoking MIL-T-FP41c when machining and fabricating I used to think beer was bad for me, so I gave up thinking. Sometimes no nonsense makes sense, in a sense. You can't teach a new dog old tricks. Honk if you're Amish
One thing you might keep in mind also is that in most rear ends there were what were called carrier breaks. That means one height of carrier would accept lower gear ratio ring/pinion and another height carrier would accept higher ratios. In the 12 bolt as I recall the break was 3.42 and down and 3.73 and up. So if you have a 4.11 rear and decide you really want a 3.07 or 3.08 rear, it may not work. Although...there are some thicker and thinner gears available which will allow you to swap without having to replace the carrier and from what I've seen those work just fine. I've also seen some folks try using washers as spacers, but I never thought that was a good idea.
Mark one of the rear tires with tape near the outside touching the ground. Mark the driveshaft with a 1' strip of tape running along the shaft near the rear U joint where you can see it while lying on the ground. Have a helper drive the truck forward exactly 1 revolution of the marked tire while counting revolutions of the driveshaft.
If the driveshaft turned just a little over 3 turns the ratio is 3.08:1. If the driveshaft turned just under 3.5 turns the ratio is 3.42:1. If the driveshaft turned 3 3/4 turns the ratio is 3.73:1. If the driveshaft turned just a little over 4 turns the ratio is 4.11:1. If the driveshaft turned just over 4.5 turns the ratio is 4.56:1.
We seldom use a GM rear end anymore but the last one I used was a 10 bolt and the gear vendors offered different thickness ring gears to avoid having to do a carrier swap at about 3.73??. I went from a 2.7 to a 4.10 and used the same carrier. Check with Gear Venders or Yukon if one is considering a gear change. We use a tach and a strobe light to get an exact read on a ratio.
I do what Bill Hanlon said but I turn the wheel 10 full revolutions. Ten revolutions will increase accuracy and allow you to tell the difference between 3.73 and 3.90 without guessing. Here’s a video I made showing the process. https://youtu.be/t4tGqJ0LZRs