3.08 to 3.42 regear, tuning question

[14burrito]

Hello Group,

 

Looking into re-gearing the Tahoe 5.3l 4x4 mom mobile from 3.08 to a 3.42 to get into a little better torque curve.

 

I have read through quite a bit of threads about gearing, sizes, cuts, 9.5 vs 9.6 etc, etc but one thing not mentioned was tuning.

 

I just got off the phone after chatting with ECGS and the closing statement from them was,

Quote

make sure you get it tuned afterwards...

 

I have re-geared Toyota's in the past and within the Toyota community, tuning after a re-gear is far and few discussed, mentioned or even thought of.

 

What is so special about the GM platform that the closing statement from East Coast was to make sure to get the TCM tuned after a re-gear?

 

I can understand the notion to ensure the tire size calibration is set properly...

 

On gearing topic,

Likely going to just run some AAM in the rear and look for a front diff assembly with the 3.42s already in place.

Anyone foresee any concerns swapping in a used assembly?

Anyone have any leads on some used diffs?

 

Thank You

 

[M1ck3y]

As far as I know you just need handheld tuner to change the gearing. Dealership can do it too, getting them to do it is another matter.

 

But, maybe there's something more involved with 2017+ models? 

[CamGTP]

Not tuning the computer can lead to many different things. The transmission can short shift or quick shift because it's seeing the mph far faster than it should. The heavy throttle mph settings will be incorrect, it might tag the rev limiter if it's too far out of whack.

 

The cluster MPH will be wrong, the cluster mileage won't report accurately, fuel mileage calculations and that will be wrong as well.

As he said above, a handheld tuner can do that change pretty easy. A shop doing the tune might be charging you a lot more money for basically the same thing.

[dieselfan1]

I have to question the slight change from 3.08 to 3.42.

Not enough difference to justify the cost 

If you do it at least go up to 3.73 or 4.10.

 

[14burrito]

1 hour ago, dieselfan1 said:

I have to question the slight change from 3.08 to 3.42.

Not enough difference to justify the cost 

If you do it at least go up to 3.73 or 4.10.

 

Stock tire size with no plans to go larger. *maybe one aspect ratio up*

 

AFM activation and fuel economy in general is much better in tow mode which is part of the reason for wanting to regear. Genuinely feel 3.08 is too high IMO.

Edited November 14, 2021 by 14burrito

[JimCost2014]

6 minutes ago, 14burrito said:

Stock tire size with no plans to go larger. *maybe one aspect ratio up*

 

AFM activation and fuel economy in general is much better in tow mode which is part of the reason for wanting to regear. Genuinely feel 3.08 is too high IMO.

3.08 is the most economical ratio in the GM line up.

 

It's just backwards from the way we think, if you have a 4.10 ratio and need better fuel mileage you would switch to a 3.42 or 3.73 gear ratio. If you have a 3.42 rear end ratio and need more towing power you would switch to a 4.10 ratio.

[14burrito]

44 minutes ago, JimCost2014 said:

3.08 is the most economical ratio in the GM line up.

 

It's just backwards from the way we think, if you have a 4.10 ratio and need better fuel mileage you would switch to a 3.42 or 3.73 gear ratio. If you have a 3.42 rear end ratio and need more towing power you would switch to a 4.10 ratio.

 

To a certain extent I can agree. But I also respectfully disagree as my fuel economy us upwards of 10% better in tow mode during daily use. Engine load. I see, both see via instant and result via tank, that tow mode puts me in a better part of my power band during driving. Sure my RPMs are higher as a result if a lower gear being chosen, but engine load plays a role as well. It appears that being at a slightly higher RPM reduces my engine load enough to get into V4 and stay in V4 much longer. Of course, driving habits and characteristics play a role and this may or may not be the case for you.

Edited November 14, 2021 by 14burrito

[MikeBMW]

I've found that 35" tires, and 3.42 gears, once tuned to the appropriate tire size, increased my city mileage from a prior 10 MPG to around 12.3 MPG.
I don't get on the highway too often so I have no input on that.
Any change in gearing, or tire size should result in an ECU tune.
More info on gearing:
https://www.motortrend.com/how-to/1211or-why-change-gear-ratios/

Rule of thumb from the above link:
(new tire [size]/ old tire [size]) x original axle ratio = new axle ratio

Edited November 14, 2021 by MikeBMW

[14burrito]

On 11/12/2021 at 9:22 PM, CamGTP said:

Not tuning the computer can lead to many different things. The transmission can short shift or quick shift because it's seeing the mph far faster than it should. The heavy throttle mph settings will be incorrect, it might tag the rev limiter if it's too far out of whack.

 

The cluster MPH will be wrong, the cluster mileage won't report accurately, fuel mileage calculations and that will be wrong as well.

As he said above, a handheld tuner can do that change pretty easy. A shop doing the tune might be charging you a lot more money for basically the same thing.

 

Thanks,

The speedo/odo being off is obvious and basic handhelds provide that option to recal. 

 

The PT sides makes sense, but it caught me off guard that I never did anything prior to on my Toyotas and the shift profiles, general PT behavior remained pretty on point before/after.