Hello, new user here and maybe someone can help?  Have a 94 K1500 suburban, has ten bolt 8.5 rear diff, but I don't know the gear ratio, sticker that used to be in glove box with RPO codes is gone, also how much if any extra work is needed if I wanted to upgrade to a 12 or 14 bolt unit?  And dont front and rear diffs ratio have to match?  Any help is much appreciated.  

they do have to match, an upgrade to a 6 lug 14 bolt 9.5 semi floater from a light duty 2500 will be the best option.

you can block the tires, jack up one of the rear wheels off the ground, put it in neutral, then rotate the tire on revolution while counting the amount of revolutions the pinion or drive shaft turns to the one revolution of the tire. 3 3/4 is 3:73  4 is 4:10......I don't know all the ratio options on the 94, there are guys that do though. you should be able to obtain it using your vin # also.  

cool tip mud i never knew you could find gear ratios that way. i think snoman will be very gappy that you endorsed the 9.5 also!  

yeah, just trying to steel the glory, LOL
I have yet to break a 9.5 axle, I have torn up some spiders and a ring gear from some major abuse though.  

Thanks Mud, for the tip I will try it tomorrow. Just bought the truck and there's a bearing out in the diff, I replaced one wheel hub bearing and seal, seal had been leaking for some time, still has bearing grinding noise coming from the rear end, dont know if it's a carrier bearing or what.  Did have some metal stuck to the magnet though.  Dont know if i should rebuild the 8.5 or try to find a 9.5 to swap.  Thanks again!  

If you are not running oversized tires on your Burb, the 10 bolt will last for a good bit. It is big tires and lifts the push them past limit. Also in the big heavy Burb I would change fluid in diff every 20 to 30K tops as it will extend its service life. Many neglect to do this.  

About the biggest I plan on using soon are maybe 235/85/R16's as it's gonna be a daily driver to work most of the time.  The 10 bolt ok if I do some light to moderate towing later on?  

I do not see a problem with that tire size. I would suggest at the very least a 4.10 axle ratio with them though and nothing taller. (A little deeper would not hurt towing)Even with a 4.10 and those tires you would only be turning just under 2000 RPM at 65 which should play well with your TBI motors torque curve. I would also suggest that you use 85w140 in rear axle if you tow in warm weather as it will give you a little more safety margin. I would not tow with 75w90 in that axle and vehicle combo in warm/hot weather. For what it is worth! One more tip if I may. If you want to wake that burb up some, Reset baseline timing from TDC to about 6 to 8 degrees BTDC and use at least 89 octane too. You will feel a difference in the seat of your pants and it will also be cheaper to run on better fuel because it will use a bit less. If you plan to stay with 87, do not waste your time advancing timing as it is set TDC to help tolerate 87 octane to begin with, not because it is the best possible setting.  

i never gave any thought to what weight gear oil i should run in my rearend. what would you suggest snoman?  

chopperfreak2k1:
i never gave any thought to what weight gear oil i should run in my rearend. what would you suggest snoman?

The really kinda depends on the climate where you live, vehicle type and the demands placed on it.  

well i'm in south western pa, the truck is a '73 chevy 1/2 ton that will soon go from d44&12bolt to d60&14bolt. it's my dd, wheeler, and tow rig.  

I would suggest 85w140 in D44/12 bolt in summer because small axles tend to run warmer with bigger tires and towing but 80w90 should work for D60/14 bolt combo  

Thanks for the info, I didn't think much either on running any specific weight oil in the rear end, and yeah it will be this summer when I do most of my towing, for instance a 20' camper!  I heard of someone swapping his 4L60 Tranny out with a beefier 4L80, do you know if they are different lengths?  

The 4L80 is a lot longer. The reason 4L60 fail towing is because of axle ratio and trying to in OD. A 4L60 has a taller OD than a 4L80 and OD clutch in it was never designed for towing loads. Run a proper axle ratio, a aux cooler and do not tow in OD and you will not have any problems with a 4L60. (BTW, a 4L60 is just a 700R4 with a electronic valve body)  

man this thanks giving i'm gonna say i'm thankful for mud & snoman. they really know their stuff don't they?