Suspect I have a mismatched gear ratio

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redfishsc

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So I discovered the other day that my speedometer is not accurate on this 1999 K 1500 Suburban. It is incrementally worse the faster I go. I can compare it to a GPS speedometer on several different devices, and it's definitely not right.

Truck originally had 3.42 based on the gu6 rpo code.

I realize it could be a faulty vss but I suspect that the gear ratio is different than factory.

I also suspect this because the rear gear hums at around 45mph, very much like ive heard on other DIY gear swaps with cheap gears.

So i took some numbers today. In 3rd gear, I'm going 48mph (GPS) at 2000rpm. 60mph at 2400, 70mph at 3000. This matches almost precisely with what some online calculators say I should have with 3.73 gears. I have stock 245 75r16 (garbage Walmart "grantland" crap).


I don't know a whole lot about the history, the friends that I got this from had got the truck from an impound lot a couple of years ago.

So how can I tell if the FRONT has been changed? I have driven this is 4wd during the snow we had, and it seemed just fine.... but could this truck (Autotrac equipped) absorb the mismatched rpm via the clutch pack in the Autotrac transfer case?
 

redfishsc

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Also, ABS works fine, no ABS light on, which usually would be on if the gear ratio were not properly retuned. So I find this a real mystery
 

RichLo

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No, the transfer case would not slip to correct mismatched gears. I could be wrong but How I understand autotrac and how it works is it keeps the front differential engaged and switches the transfer case between 4wd and 2wd when it senses wheel spin. If this is how you drove it in the snow it was probably ok because it's only in 4wd when there's wheel spin.

However full time 4wd would blow the chain inside the transfer case on dry pavement. your best bet would be to find some loose gravel to try 4wd. Drive in a straight line slowly and if it drives and sounds normal your gears should be the same. If it feels sluggish, comes to a stop fast, bounces around like the front and back are fighting each other and sounds like the tires are moving the gravel your gears are defiantly mis-matched.

If they are mis-matched I would make it impossible to have 4wd work. un-plug the front differential actuator so you cant accidentally blow up the transfer case (Or somebody else if you let somebody else use it).
 

90halfton

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The quickest smart way to check is to jack up your truck and spin the tires. Count the number of turns of drive shaft vs. tire. 3.42 should net you about 3 1/2 turns of the shaft for every tire turn. 3 3/4 turns would be 3.73, etc. Mark tire and pavement, and yoke and differential housing to easier count turns.
If you suspect they're different for f###$ sake don't try driving on any surface in 4wd.
 

redfishsc

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I drove it in 4hi (not auto) a good bit when it snowed and not all of it was straight ice, so I think I'm OK based on what you guys are saying.

I'll jack it up and test both diffs to see what the ratio is.
 

magimerlin

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Probably due for a diff fluid change anyways... sooooo. Take the diff covers off and see what the ring and pinion have for numbers stamped on them.... let's you know 100% what ratio is in them. Yes the rotate tire check works to but like I sad.. most likely time to change the gear oil anyways...

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90halfton

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Probably due for a diff fluid change anyways... sooooo. Take the diff covers off and see what the ring and pinion have for numbers stamped on them.... let's you know 100% what ratio is in them. Yes the rotate tire check works to but like I sad.. most likely time to change the gear oil anyways...

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This is a very good idea
 

redfishsc

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I have the rear diff cover off today, I forgot there may be numbers stamped in it.

Will check it and refill with stank oil after church today.
 

Biggershaft96

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If it moves in 4x4 then ratios are the same or something isnt engaging. You would know something is wrong if they were mismatched the second you went into 4x4.
 

redfishsc

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Ok well I pulled the cover off, it's clearly marked 12/41 which is definitely 3.42. Unfortunately it looks like they are replacement gears (unknown brand, didn't think to look) so I checked the gear pattern with some white lithium, and it seems they have the Pinion depth too shallow. The cover didn't have excessive metal dust on it so I will just deal with a little gear hum.

So now i have the task of figuring out why the speedometer is wrong.


Both diffs were at half capacity. Rear had stalactite of road crud, RTV, and gear oil growing off the bottom. They had the cover glued on (poorly) with RTV which was why it leaked, they basically torqued all the RTV out of it. I put a Felpro on it, all tight and happy.

I don't know where the front was leaking, I guess I'll find out. It was half full of black, spent oil.

Next up, transfer case.
 
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