Speedometer Calibration tab and shift points, etc (E38)

[jamesr]

I've done a lot of searching and haven't ever found any real explanation of how the ECM/TCM use these values. It seems most of the tunes I've looked at lately including my 2008 sierra have the final drive ecm and tcm ratios mismatching, and the VSS tire revs per ecm/tcm also mismatching, but not in any way that correlates with each other.

Anyone have information on how these values work and any math to make sense of it? I've tested modifying the trans ratio and dist-trans values and both have an affect on my shift points.

I've attached my 08 sierra stock settings and a 2007 avalanche i worked on yesterday. Doesn't seem like there's much rhyme or reason to it (at least not obvious). If I change the TCM values to match the ECM and blow some new values in the shift points tables with the bluecat tool it seems to be spot on, but I've heard of people having ABS issues modifying those values so would like to understand how they work.
2007avalanche.png
2008sierra.png

[jamesr]

To make it even weirder, none of the data on the speedo calibration tab matches up to my stock tire size anyway. I haven't found any calculation that makes any sense of any of it.

[jamesr]

It seems like nobody has any real clue why it's like this, but here's what I have messed with and the results:

Driven and Non-Driven tire circ: Does not seem to have any affect on anything easily noticeable. I put them down to like 7, up to max, and split one min one max and nothing happened.
Ratio and Dist (ECM): Adjusting either of these changes the ECM's calculated speed. Anything in the ECM based off vehicle speed, including the scanner and speedometer displays seems to be affected by this. No affect on shift points or anything trans related.
Trans Ratio and Dist - Trans: Adjusting this does not alter anything engine/speedometer related on the ECM side, but alter's the transmission's vehicle speed value, which affects shift points, lockup, and anything else speed based in the trans.

Best explanation I have for why they don't match is that GM doesn't care and didn't want to calibrate them all exact for every vehicle. The shift points are low enough it doesn't matter. However I used the bluecat tool to set my shift points based on my ECM side's values and I would overshoot the RPM value by a few hundred and hit the rev limiter. Once I made the trans and engine agree the shifts are spot on. Have not had any sort of ABS light kicked off as I've heard could happen, but I don't have big tires, though I'd think somewhere in my testing it might have thrown something off. Some of the calculated differences i've seen are pretty dramatic, especially with the 6 speeds so there might be something else to them. Shift points are probably set from the factory based on whatever trans values are in there anyway.