T42 Speedo calculator help

[Brads_rs]

hi, maybe i am over thinking things or overlooking something. Problem is i am trying to change the tire size and after i put in the value that i want (27.90) it comes up with some crazy high number. I thought it was strange when the stock value was 6.12. When i plug 27.90 in to the shift point correction table it wants to adjust by 300%, which set the shift points way to high. This is my first T42 tune there must be something that i am overlooking. I searched the forum with no luck. Any direction would be great. I included a copy of the tune file below. Thanks in advance

[Taz]

Hello Brad,

The ECM / TCM programming as it relates to tire size, speedometer calibration, and shift point correction, in GM trucks varied somewhat between 2007 and 2010.

Are you able to post your original (unmodified) ECM and TCM tunes, as well as the size and manufacturer of your original tires, and the tires you plan to use ?

Sometimes the ECM contains part of these parameters, and the TCM contains the rest. Other times, both the ECM and TCM have all parameters, and the “necessary” tire diameter ends up being miniscule (i.e. the 6.12 inches you mentioned) as the final drive ratio (e.g. 3.73 gears) seems to have been “double counted” or factored into calculations twice.

If you don’t want to post all of this information, feel free to send me a PM.


Regards,
Taz

[Brads_rs]

I don't have the original tune, the ECM is from a 08 uplander and the tcm is from an 08 truck. I flashed a 07 trailblazer program in to the ECM and the TCM is still stock. What i am doing is putting an ls2 into a 55 chevy for a customer. I have the harness built and contacted my customer for tire size and gear ratio, which was 3.73s and a 27.9 tire dai. The 07 blazer program has been modified and i have already changed the tire size and ratio in it. I am use to the old ls1 411 pcms which seem to be so much easier to work with. I will post the ecm tune file for you. I will see if i can get the tire size from the customer. As i said before this is my first time working with this combination, so please be patient with me :grin:
Thanks, Brad

[Taz]

A "mix and match" is always challenging. If you could post the tires that are going to be used (ie. BF Goodrich TA P265/50/16) that would be helpful. I prefer more information than just the diameter - and will confirm specifications using the tire manufacturer's data.

The ECM from the 2008 Uplander is an E67, that would have originally had a V6 calibration. Did you flash the 2007 Trailblazer tune into this ECM using a Tech II or similar device ? An EFILive flash (even a Full flash - Calibration & OS) is not sufficient to repurpose the ECM from a V6 to a V8 application. I mention this as you indicated your "comfort zone" was the LS1-B PCM.


Regards,
Taz

[Brads_rs]

I emailed my customer today hopefully i will know some tire info soon. I did not use a tech II to reflash to the v8 but i can have this done. I was thinking of doing that anyway as i wasnt sure if the full flash with efi live would work. I will try to have it flashed tomorrow, and post the new tune files.