I have the 14 bolt, heavy duty G80 but do not see a ring and pinion ratio code. Does anyone know the various codes for gear ratios in this tranny?

Thanks.

 

You should have another Gxx code that indicates the rear end gear ratio. For example, my 1996 has the code GU6, which decodes as a 3.42 rear axle gear ratio. My 2006 has GU5 for a 3.23 ratio, as well as the G80 to indicate the locking rear diff. I wish my 1996 had the G80 as well, but it is what it is...

Not sure this is 100% accurate, but I found this list on the Internet:

GU2 = 2.73
GU4 = 3.08
GU5 = 3.23
GU6 = 3.42
GT4 = 3.73
GT5 = 4.10

 

Thank you. Yeah, I found it odd. The code labels were all worn off, so I had to pay the $50 to get the original build sheet. It showed the G80 code, but nothing else. I am trying to get the info from both the GM Heritage and Eaton people. Hopefully someone will come thru.

 

Have you tried the VIN decoder, in the Tools menu at the top of this webpage? That may tell you the axle ratio...

 

As a followup, I tried the VIN decoder tool at the top of the page myself, with my 1996 C1500 VIN. It listed the proper engine, but did not specify the equipped rear axle ratio - just a RANGE of available rear ratios (3.08, 3.42 and 3.73) - mine has the 3.42.

I imagine with a C3500 it may have been limited to a single rear end, since those are MEANT for heavy duty use and towing. I suggest the VIN decoder just in case it turns up any useful technical info.

 

Yes; it states {GT4} 3.73 rear axle ratio. However, I don't necessarily believe that because so many other things are wrong. There are no two sites that agree.

 

If you’re still needing an answer….

Are there any ID tags attached to the axle? Oftentimes they are held on by one of the cover bolts and will have the ratio stamped in it.

If not, try calling a driveshaft shop - if they can’t tell you immediately, they probably have the technical resources to find out.

One other thing would be to get the rear wheels off the ground, mark the driveshaft and one of the tires, then put it in neutral and count the # of driveshaft turns to one revolution of the tire.

If I’m not mistaken, I think the 2WD G80’s of this era were more commonly equipped with a 4.56 ratio, so if you get 4+ turns of the driveshaft, that should be a clear indicator.

If not, and you can’t find it elsewhere, the only way to verify would be to remove the cover and check the stamping on the edge of the ring gear. That, or count the # of teeth on the ring gear and divide by the # of teeth on the pinion,

 

Thanks for the response. The Heritage people did come thru. They found other documents associated with my particular build, and sent them to me by email. Digging into them turned up the codes G80 and GT5. This means that it is the Eaton Locker (GovLock) with the 4.10 ratio.

I thought the engine was running a little fast. Now I know why. The only solution is to find a G80 with the other ratios; maybe one with the 3.21 ratio, since there appears to be no aftermarket replacements available, nor originals either.

 

My understanding is that you WANT the higher axle ratio for pulling a heavy load - like a 5th wheel if that is what you are doing, as it transfers more torque and has a higher towing capacity than the lower ratios. If you drop from 4.10 to 3.21, you will be cutting 1000's of pounds out of your towing capacity. My 1996 manual and my 2006 manual both show higher towing capacity with the higher ratio rear ends, and it drops as you go down - significantly.

 

Yes, I do understand that. The thing that many people do not understand is that getting the load moving is also a determination of the transmission's lower gears. However, at the higher end, it is getting increasingly difficult to maintain highways speeds because of the tall diff gears. Also, a diesel engine develops lower speed torque, and that might be a consideration in the equation as well.