2015 Tahoe change rear gear ratio

[Telco]

Hello - I have a 2015 Tahoe with 3.08 rear gears that I am about to have replaced with 4.11s. I've found that H0122 in both the ECM and TCMs are the gear ratios, and have changed them. However, my experience with EFI Live in the past (Gen 3 4-speed stuff) is that when the rear ratios are changed there is a way to recalculate the gear shifts for the new ratio. I did find the one that recalculates if I change tire sizes. Am I missing something here, or will the gear shifts have to be manually changed for the new ratio? I checked around the site and didn't find any tutorials that address rear gear changes, but did find a tutorial for changing the shift points so if nothing else... at any rate, am I missing the conversion calculator? I figure I can also play with the rear tire size to correct the shift tables, then put the correct tire size in with the new rear ratio. Mainly I want to make sure I'm not missing something since this is an unfamiliar system to me.

[Telco]

Decided to clarify what I'm looking for more simply, since I tend to get wordy.

What I am looking for is how to recalculate shift points when the ratio is changed. Is there a calculator or does it require manually adjusting for the new ratio?

If nothing else, thinking that if I go in and multiply all the shift tables by .7493 that this should adjust the shift points to where they need to be for the gear change. This way makes me nervous though. For now I just want it to shift like it would if it came from the factory with 4.11s, I can tune for preference later.

[Telco]

OK, I think I figured this out, and it wasn't very straightforward. If you missed any of the steps it didn't do anything, and there was no indication on what field was missed. I would like one of the gurus to verify my procedure though, and if it's wrong I will delete what I'm putting on here to prevent the dissemination of misinformation. It did adjust the ratios by the approximate amount I needed it to go though, so I think I have this correct.

To change the rear gear ratio on a 6L80E transmission -
1. Go to Transmission Calibration, Speedo, Parameters.
2. Select H0136, Tire Revolutions.
3. At the top pick the Speedo Calculator. A new box will open with 4 tabs across the top.
4. On the Speedo Calculator tab, adjust the Final Drive Ratio (H0103) to the ratio you want. Hit Apply.
5. Go to the Shift Point Correction and enter the old and new rear end ratio. When you put the two ratios in, a percentage should appear in the Adjust By field. Hit the Select All box to make sure all fields are adjusted, then select Apply. Only hit Apply once, and it will adjust all fields by the percentage in the box.
6. Change field H0122 to the new ratio. I don't know if the program uses this field for any calculations or not, but since it's there make it correct. Save your program.

There is also a field in the ECM that reflects the axle ratio, and it is also field H0122. It will need to be changed as well.

[joecar]

Thanks for posting... yes, that's how to do it... when you go for a test drive, does VSS agree...?

IDK either where/what H0122 is used.

[Telco]

No idea yet. I just told the shop over the weekend to get the parts together. I'll post back the results when it's done but it may be a couple of weeks. Thanks.

[Telco]

Tahoe is at the shop getting the gears in now. I went ahead and loaded a cal to the engine and trans, and when I left the shop there was a Reduced Engine Power warning on the DIC. Revving it in neutral does not indicate that it has a normal power level, revving is much slower than normal in neutral. I THINK I may have found the problem already after some searches and will be going down later to install the updated tune with factory limiter values put back in. If someone can take a look and make sure I'd appreciate it. These are my corrected programs.

[Telco]

These two are what is loaded now, and gave me the reduced engine power alert. I'd appreciate fast action on this one if anyone is available and willing to look as they expect to have the work done this afternoon. If at all possible I'd like to have a working program in for them to do the break-in work. Much thanks in advance.

[joecar]

I'm looking at your files...

[Telco]

Excellent, thank you sir.

[Telco]

Update from the shop - they have the new gears in, and did a break-in run. The DIC is still displaying the reduced power alert but he advised that the Tahoe didn't act like it was down on power. They are closed now, I didn't have time between them getting done and them closing to pick it up. So, it's not so critical to me to find out why the reduced power alert as I thought it was earlier. Based on the searches I did before, this points towards the problem being the max speed being set too high, possibly putting it into conflict with OnStar although I don't understand what the connection might be. If this is the case I'll need to reduce the max allowed speed. It's not like the Tahoo would ever go 300MPH+ unless I mounted a 747 engine in the back. :mrgreen:

I'll drive it a few days, then update how well the shift pattern is working with the procedure I put in. Thanks all.

[Telco]

Got the Tahoe back. It still has the Reduced Power thing when you start it, but power is not reduced. I'll try backing the speed limiter down to 255 sometime today and see if it's still an issue.

The new gears feel great. It still feels like I'm driving a big box, but the box isn't as big as it was.

And now, for the important bit - the procedure for updating the shifting profile for a gear change works perfectly. This should work for tire size changes too. Aside from the shift speeds being lower, the Tahoe's shifting pattern feels identical to before the gear swap. If you moderator types want to copy it to a fresh thread and make it a sticky, be my guest.

[joecar]

I can't see why it is throwing a REP DTC...

[Telco]

Thanks for looking into it. I've not had a chance to make any changes yet and it's nighttime here. I'll try to get something with it tomorrow and will report back. Going to try putting in 255/254, like the older computers had as a limit and see what that does. With the new gearing it's best theoretical speed is 207MPH but there's no way a stock 5.3L will drive that huge box that fast. I'll report back the results after.

[Telco]

OK, the error is gone. I went to the engine calibration, Speedometer, Speed Limiters, Parameters, and set H0129 to 240, H0157 to 239, H0158 to 239, H0161 to 238, and H0131 to 239. No other changes made.

I suppose someone could sit down and load ever higher speeds in these fields until the error returns and possibly identify the cause from there, but as for me the error is gone and the speed limiter is set higher than the vehicle's actual top speed so I'm happy with it as it is. It's not like it'll ever go 200+MPH anyway.

[joecar]

I don't understand why... but good job :cheers: