'03 Sierra 8.1/Allison tire size gear ratio change issue.

I picked this truck up a few months ago. It has 285's on it from the PO. I'm not trying to do anything tuning wise other than tire size correction and gear ratio change. I initially changed the tire size to correct the speedo using the wizard and everything worked as it should. The rear pinion bearings took a crap shortly after from when the PO replaced the pinion seal and I'm guessing he got the preload wrong. I have done enough to this truck since I bouth it so I just wanted it back on the road. I picked up a replacement from a truck that had 4.10's from the factory (GT5 RPO) and foolishly didn't open it up and check the ratio. It is a 3.73 rear. I went forward with swapping it in and used the wizard to change the ratio. This time around it made the trans hold gear like it was in tow/haul and would't lock the torque converter. I put the stock tune back in and everything is back to normal except the speedo of course.

Now that I'm thinking about it more while I type this. I don't think I wrote the TCM for the tire size change due to the several attempts to write the PCM before it finally didn't time out and completed the rewrite. The second change for the axle ratio I ended up writing the PCM and TCM seperately due to time out issues.

So I think I answered my question but need some verification. Do I need to write the PCM only for those simple changes and leave the TCM out of it?

For later cars you'd need to write both. I'd assume that goes for this one too.

half of the speedo stuff is ECM the other have is in TCM

So I put the factory tune back in and it shifted like it was supposed to. I decided to give it a shot with only writing the gear/tire size changes to the PCM and not the TCM. Everything is as it should be. Properly timed shifts and a correct speedo.

Those separate Allison TCMs are kind of wonky. I usually don't change anything in them.

Yeah, I figured that out the hard way. I've never dealt with one that had a seperate TCM.