Ok, I concentrated on some XGauges that seemed like they might be working right. Specifically, Input Shaft Speed (ISS) and Output Shaft Speed (OSS). They looked good testing in my driveway after dividing OSS by 10.
This should tell me without any doubt what gear I'm in and (comparing ISS to engine RPM) how much TCC slip I've got.
The good news is that they seem to work right -- they pass sanity checks (not reading 20,000 rpm or 2 rpm) and change as expected (following speed, gear changes, and eachother). The bad news is that they're both reading too low by ~400 RPM.
I've calculated what my OSS/driveshaft RPM should be at 70mph and at 2100 engine RPM. It should be reading 2800-2900, but reads 2350.
There are 3 ways ISS should compare to engine RPM:
ISS < engine RPM: TCC unlocked + engine applying power
ISS = engine RPM: TCC locked, and/or transmission in Neutral
ISS > engine RPM: TCC unlocked + engine braking
In all conditions it reads lower than engine RPM (
) but the amount varies based on which condition is in place; it appears that it will read correctly if I merely add a few hundred RPM before displaying it.
The only thing that makes me doubt that is that I read ISS of 800 at 580 engine RPM...but maybe I was confused when I wrote that down.
theholycow 
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