Mismatched gear ratios in differentials M998

[AOR]

I have a 1986 M998 that was overhauled in 2007 in ME. I noticed on the diff cover tags that the rear is stamped 2.56 and the front is stamped 2.73. I was going to pull the covers today with the vehicle on a lift until I discovered the brake caliper mounting bracket is blocking access to one or more of the cover bolts so I marked the pinion yokes and half shafts and had someone hold one wheel while someone else watched the drive shaft while I spun the other wheel. We tried this various ways and came up with the same results. I was hoping someone put the wrong tag on one of the covers but it is clear that one pinion yoke turns more than the other with 1 revolution of a half shaft. Has anyone else seen this where they have mismatched differential ratios? I have not noticed any binding but I also have not driven on pavement in high lock mode. The HMMWV has the NP218 transfer case which I have been told by various sources does not use clutches in the transfer case like the 242 models do. I have driven Jeeps with mismatched ratios with wider spreads in the ratios and in 4x4 mode they made terrible popping sounds from the transfer case

 

[NDT]

That is certainly a quality control issue, but as far as the truck is concerned, the mismatch is allowable as the transfer case has an inter-axle differential. When HL or LL is engaged, the inter-axle differential is locked, and if you are on a high traction surface, the truck will pop and jump like you describe.

 

[Retiredwarhorses]

You never want to mismatch diffs....I had a truck in the shop recently they had this same issue.
installed a 2:73 to square it up...2:56’s are bit harder to find, 2:73’s are plentiful.

 

[AOR]

You never want to mismatch diffs....I had a truck in the shop recently they had this same issue.
installed a 2:73 to square it up...2:56’s are bit harder to find, 2:73’s are plentiful.

Thank you I have never seen anything come from a manufacture with mismatched ratios. What symptoms did they have and when were they able to create them? I had no idea the ratios were different since it drives smoothly
I am considering installing ARB lockers so I guess I can just change the back to 2.73 to match the front at the same time. I was going to put it in HL and drive it in the rain and see what happens

 

[jasonm43]

so here is a question for the gurus, I am doing an L86 powertrain swap and one of the benefits is I will be able to do a digital speedometer. The PCM programmer would like to know two things, Tire size and final gear ratio. Any thoughts on how I should calculate with the reduction gears and the diff gears?


any and all help is appreciated and this forum is a huge help, these are our first two builds and the next one should be insane

 

[Skrilex]

You add the differential gear and the hub gear together to get final gear ratio. The hubs are 2:1 I believe so it would be 2.73+2= 4.73:1

EDIT: For example of dumb, see above. You multiply the diff gears by the hub gears. See below.

 

[Keith_J]

You add the differential gear and the hub gear together to get final gear ratio. The hubs are 2:1 I believe so it would be 2.73+2= 4.73:1

It is the product, not sum. Multiply 2.73 by 2 for a overall gear ratio of 5.46:1 .

 

[Skrilex]

It is the product, not sum. Multiply 2.73 by 2 for a overall gear ratio of 5.46:1 .

Dang it you’re right I knew that looked wrong.

 

[G-RoT]

You add the differential gear and the hub gear together to get final gear ratio. The hubs are 2:1 I believe so it would be 2.73+2= 4.73:1

EDIT: For example of dumb, see above. You multiply the diff gears by the hub gears. See below.

 

[Mogman]

RWH is 100% correct, you do not want mismatched diffs, the spider gears in the transfer case are working overtime to match the load and will build heat and wear really fast!

 

[G744]

Mismatched gear story: A friend bought a new Chevy 2500 4X4. The gears were wrong. The dealer said the axle ratio tags were the same, so he drove it around the parking lot a bit and the transfer case cracked open. Warranty fixed it, happy now.

DG

 

[rakhav]

Thank you I have never seen anything come from a manufacture with mismatched ratios. What symptoms did they have and when were they able to create them? I had no idea the ratios were different since it drives smoothly
I am considering installing ARB lockers so I guess I can just change the back to 2.73 to match the front at the same time. I was going to put it in HL and drive it in the rain and see what happens

i have a 1987 m998.. it recently had rear end isssues.. the number in the back is 5579423.. i was told that is a 2.73 ratio... if i purchase another rear end witht the same number 5579423 would that be correct .. I was told the earlier models cam with a 2.56.

 

[rakhav]

You never want to mismatch diffs....I had a truck in the shop recently they had this same issue.
installed a 2:73 to square it up...2:56’s are bit harder to find, 2:73’s are plentiful.

Thank you I have never seen anything come from a manufacture with mismatched ratios. What symptoms did they have and when were they able to create them? I had no idea the ratios were different since it drives smoothly
I am considering installing ARB lockers so I guess I can just change the back to 2.73 to match the front at the same time. I was going to put it in HL and drive it in the rain and see what happens

can someone help me please.. i have a 1987 m998 .. the rear is making some terrible noise,, I was told that the rear diff is fried.. The part number that is stamped on it is 5579423 ... This part i was told is a 2.73 gear ratio. I was also told that the earlier models came with 2.56 gear ratio. since the old diff on my humvee is 559423 , is it safe to just install another 5579423.. txs..

 

[rakhav]

I have a 1986 M998 that was overhauled in 2007 in ME. I noticed on the diff cover tags that the rear is stamped 2.56 and the front is stamped 2.73. I was going to pull the covers today with the vehicle on a lift until I discovered the brake caliper mounting bracket is blocking access to one or more of the cover bolts so I marked the pinion yokes and half shafts and had someone hold one wheel while someone else watched the drive shaft while I spun the other wheel. We tried this various ways and came up with the same results. I was hoping someone put the wrong tag on one of the covers but it is clear that one pinion yoke turns more than the other with 1 revolution of a half shaft. Has anyone else seen this where they have mismatched differential ratios? I have not noticed any binding but I also have not driven on pavement in high lock mode. The HMMWV has the NP218 transfer case which I have been told by various sources does not use clutches in the transfer case like the 242 models do. I have driven Jeeps with mismatched ratios with wider spreads in the ratios and in 4x4 mode they made terrible popping sounds from the transfer case

can you call me please .. i have the similar issue.. just wanted to touch base with you and see how things worked out for you. Thank you. retired cop.. rocky

 

[Maxjeep1]

can you call me please .. i have the similar issue.. just wanted to touch base with you and see how things worked out for you. Thank you. retired cop.. 867 5309 rocky

I would delete your phone number. Private message him and a retired cop should know better than to put it on a open forum

 

[AOR]

I still have a good diff I think its the 2.56 or whatever was numerally lower I can sell to you if you have the same ratio I see you have a 703 number so you are not far from me I sold the vehicle the issue I had was hard to explain I felt the driveline binding and it whined more than expected the noise was from the transfer case it was not happy also dont go by the tag number go by the numbers on the gears or count teeth

 

[rakhav]

I would delete your phone number. Private message him and a retired cop should know better than to put it on a open forum

Sorry. How do you do that. New to this site

 

[Coug]

if the rear end is fried you aren't driving it anywhere.
Either jack it up and rotate the wheels and count driveshaft rotations to figure out the gearing, or pull the differential out and pop the cover to check and count all the teeth on both the ring and the pinion to figure out the ratio.

Sometimes the wrong tag ends up on the wrong piece of equipment, or sometimes if a motorpool needs parts and they have a truck that's getting disposed of, they'll take whatever parts they need, and toss whatever the worst parts they have into the disappearing truck and call it good. Not their problem after that.

Without physically verifying what the gearing is, it could be anything.

 

[Coug]

Sorry. How do you do that. New to this site

At the bottom of your posts there should be an "Edit" option


right about here
-----------VVVV

 

[Maxjeep1]

Sorry. How do you do that. New to this site

Click on his username and it will show start conversation. You might not be able to do it if you are new new