M1008 Speedometer recalibration

[doghead]

Now that I have 37" radial Humvee tires on the M1008, my speedometer reads 50 mph and I am actually going 59 mph. That's a 20% error at 60 actual mph.

I found the add on adaptors, that go in-line with the cable. I can order one with any ratio correction I want. ($70)

Also, I assume I may be able to change out the speedometer cable, driven gear in the T-case. While researching this idea, all I find is the gears for in the transmission housing,(not in a T-case). (less than $5)

Has anyone here gone with 37" tires and re-calibrated your speedometer to indicate correctly? If so, how did you do it?

 

[Ken_86gt]

I believe all you really need to do is change the driven gear in the t-case (the one that connects to the speedometer cable). Look here, there is also a calculator to choose the correct gear tooth count.
Speedometer Gear, Speedometer Gears, Ford, GM, Dodge 700R4

 

[gimpyrobb]

Just hook up the gps unit and use the speedo function!

 

[doghead]

Ken, I planned to call that place today.

What I don't know is, if the T-case uses the same gear as a TH400 transmission. Also, I will need to know which Drive gear is in my T-case. It would also be nice to know what driven gear I have(without pulling it out first).

Has anyone here done this on an M1008, and know these things?



Gimpy, using the GPS is how I checked the speedometer error.

 

[Crash_AF]

The drive gear in the T-case should be a an "A" gear per the Chevy Truck parts book I have. That translates to a 7 tooth drive gear. The Driven gear should be a "U" gear which is 21 tooth. (ETA: This info is for a 205 transfer case...)

ETA: The 208 should have a 15 tooth "B" gear as the drive gear. There is no data for a 208 with 4.56 axle gears as that was not an RPO except for the CUCVs.

Where did you find a driven gear for $5? I need to change my gear to a 40t since I went down to 4.11 gears.

Later,
Joe

 

[doghead]

Like a fool, I didn't save my search results! I am not sure where the $5 gears were, now!

Looks like they are only $11, on the link above.



If the drive gear is a 15 tooth, then I think I need a 37 tooth driven gear to make mine correct.

I have a call into the PATC, site above to see if I can confirm this, before I order.

Ok they called back. there are two driven gear housings. they each have a series of numbers on them like, 33,34, 35 or 40, 41, 42, 43. These numbers should be visible when looking on the opposide side of the gear housing cover, that is not covered by the retaining plate.

I need to go look at mine and see what it is.


btw, the driven gears are the same as for the TH400, for the NP208 T case.

 

[doghead]

Well mine has a 40, 41, 42, 43 on the cover. That means I need the other driven gear housing to use the 37 tooth driven gear. That's another $56.

They did mention that the 15 tooth drive gear is no longer available. Also the external adaptor puts more stress on the gears, so it is not the preferred method to recalibrate.


I also found info that says you line up the number on the gear housing with a mark on the T-case.

So when I have the correct housing for a 37 tooth driven gear, I will line up the 37 on the cover, with the mark on the T-case.

 

[Crash_AF]

I just picked up the 40 tooth gear from the local dealer for $9.19 +tax. They had one at the local warehouse.

I'm not surprised that the little eccentric sleeve is 40-43 tooth as the stock gear is probably 42 or 43t. You rotate the housing to engage the teeth of the gears so that the different diameter gears can use the same housing. Try going to a pull-n-save and getting one from a civvie truck. AFAIK any 208 cover will work from a Ford, Chevy or GM.

I have GM part numbers for a 36 and 38 tooth gear:
1359270 is for a 36t gear which is White
1359272 is for a 38t gear which is Blue

Given that the number jumps two digits, you could conjecture that 271 is a 37t gear, but I cannot confirm it with the book I have.

For reference, the GM speedo chart that I have says that a 38t gear is used with 235/75R16 tires and a 3.73 axle ratio and a 36t gear is used with 215/85R16 tires and the same axle ratio. If you can run those numbers through your speed/tire/gear calculator to get the same RPM range as what your speed/tire/gear combo is it would nail down which gear you need.

Finally found the number for the housing, which GM calls a 'sleeve'. The part number for the 36-39 sleeve is 1362294.

Later,
Joe

 

[doghead]

Well, I installed a new 37 tooth driven gear and new correct housing, today. It is now indicating about 2.5-3 mph too fast now.(used a gps to verify speeds.

I'll be ordering a blue 38 tooth gear next. That should get me where I want it.


btw, I did not find any mark to align the gear tooth number with. The retainer that holds the housing into the T-case can only work with the housing in one position.

 

[motormayhem]

Any ideas on what I need if I have a m1009 on 36s and regeared to 4.11? 65=45 right now.

 

[doghead]

Use the link in post number 2.

 

[cjtroutt]

What T case do you have NP 208 /205 . the 208 in my 1008 went bad was an 208 its an easy fix most trans shops might still have one in there back room.
I still think your first track was a better choice with transducer to re cal .od.
My speed cable was melted on exaust pipe I re routed it down driver side loop over T case.

 

[Crash_AF]

Have you pulled the little converter box out of the system yet? That will at least help get it closer.

Later,
Joe

 

[doghead]

I installed a blue 38 tooth speedometer driven gear. I am now reading within 1 mph of my actual ground speed!

 

[dstang97]

i was just going to ask you about it. thanks.

 

[Recovry4x4]

Whatever you used Eric, bring a second set to the GA rally. Wait, nevermind, the 700R4 will screw all that up!

 

[AJMBLAZER]

I need to do this. Mine's WAY off...but if I go 45 I'm doing 55. It evens out.

 

[dstang97]

I took out my green gear and put in a blue one, now I have nothing. It seems like the gear is to small to pick up the other one?????? what gives????????

 

[Ken_86gt]

Read through the thread again. There are rotation marks to line up for teeth count and different housings for different gears.

 

[AJMBLAZER]

Yeah, sounds like you have the wrong housing for that gear,