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gearing. To change or not to change...and will the abs and components still work?

4K views 8 replies 4 participants last post by  altamate  
#1 ·
I have a 2007.5 lmm 2500hd, duramax w/6sp allison, 6 inch lift on 35's. quite a bit of work done, found it was very hard to get the speedometer back to normal because the abs would get upset after changing the ratios via efilive. The abs is not programmable, third gen was, fourth is not. I have access to a tech 2.

What I would like to accomplish is to change the front and rear diff gear ratios to get back to a stock final drive ratio, maybe a little higher but either way close to stock final ratio.

after finding out how sensitive the abs is to changes in the ecm speedo ratios, is it possible to change the gearing, and change my ratios in the tcm and ecm, and retain the abs system? I very much want a functional speedometer, but dont want the light associated with a pissed off abs system.

All feedback is appreciated. I need to study this before I dive in, changing the gears is easy but fixing the lights is much harder!
 
#2 ·
Moved to drivetrain
 
#3 ·
Did you regear - or are you just trying to fix the speedo?
In EFI Live there is a setting for tire revolutions per mile. This is where you "fix" the speedo. See if the tech2 has a similar setting.
 
#4 ·
I have changed the settings in efi live to compensate for the larger tires. I would like to regear the truck to regain some of my low end power, I.E. lower gear ratio in the front and rear differential.

When I lifted it the speedometer became inaccurate, via efi live I recalibrated the speedometer, but when doing so the abs became upset since the ecm was saying 70 @ 70mph and the abs says a different # since it was built with 17s.

I had to dial back the tire revolutions to get the abs to quit flagging the c0245 in the abs. It is still fairly accurate 67 on dash @ 70.

But when I change the gearing it will have a dramitic change in wheel speed. I would like to maintain some accuracy on the speedometer

You show a LMM and a LBZ. The lmm no longer has any abs changes where you can alter tire size, the LBZ still has the ability to go as large as 34.5.

Ultimately what will happen is I will change the settings in the ecm to compensate for the larger tires, and the gearing to correct the speedometer. What I am trying to identify is if anyone has done this without getting the c0245 code....
 
#6 ·
Unless something changes, there are 3 abs sensors. One on each front wheel, and one at the tcase output. The ratio of the front to rear is the diff gearing.
As for as speedo, look at revolutions per mile (H0136). Check the tires you have on the internet, for the revolutions per mile and plunk it in - leave everything else stock.
As far as the TCM, leave it alone unless you think you need to change something. All it knows is output speed, which hasn't changed.
 
#7 · (Edited)
You have 2 choices to slove your problem with the speedo.

1. If you are running efi-live you can program the tire size to match the 35" tires you are running but there is a small problem. Max size allowed is 34.5" and on top of that, if you actually program 34.5" in you get the service ABS or trailer braking system codes. This is becuase even though you can cange the tire size in the system to 34.5", it does not like it and throws the codes. The work around is to enter a slightly smaller tire size. You are then close enough that the speedo is correct or barely out and no codes (win win). Use 33.9" in the computer and you will find that your speedo will be out(low) only a few MPH at 70mph.

2. The other solution is leave the programing alone and switch from the stock 3.73 gears in the truck to either 4.10 or 4.30 with 35" tires. If you do 4.10 and 35" tire, your speedo will read ~70 at 73-74mph(almost identical to the first solution). If you do 4.30 your speedo will read ~70 at an actual speed of 66-67mph.

I have done both solutions on my 2008 and both work.
 
#8 ·
This is exactly the information I am looking for. I have already corrected via efi live, but I want that bottom end back, so I am going to do some gear changing. I do not have jl4 brakes which has all 4 wheels individual wheel speed sensors, I have the simplier system, front sensors, and ecm communication.

When you did your gear change did you have to switch the carrier(part ring gear attaches to)? sometimes with deeper gear swaps the carrier is not offset correctly. I would likely go to 430s.
 
#9 · (Edited)
Had a shop do it. Pretty sure they used the 3.73 carrier for the 4.10s or I would have paid for the new carriers too. They offered me the option of 4.30 or 4.56 for the same money, so I think the 3.73 carriers work for all these ratios.