The ABS module outputs a speed signal over the CAN, and also wheel slip information. The speed information is used by the Ipack (for the speedometer) and the CCU (inhibit rear door release) the CCU also sends out a PWM pulse based on road speed for the audio system (to alter sound volume based on road speed. I suspect that the ECM and TCM will also monitor the CAN for vehicle speed information, but I'm not 100% sure what they use the information for. The wheel slip information is used by the ECM so it can ignore wheel spin vibrations from variations in speed pulses from the crank sensor, which could be introduced by the wheels slipping and gripping.another question.
The ABS controller can see both engine revs and wheel speed.
Maybe it's the ABS controller that decides whether you are in first / reverse or some other gear?
That relationship changes with different final drive (IRD) ratios so I wonder if the ABS controller is calibrated differently for this?
An what happens when you fit a non standard gearbox / IRD ratio?
I think that the only consequence would be that maybe it will lock me out of hill descent because it will never accept that the car is in first gear. Probably not a big deal for SWMBO
I don't believe the ABS module knows or needs to know the engine speed, but it does need to know throttle position, so it can instigate HDC target speed and possibly TC if it sees excessive throttle for a given wheel speed.
Ali has found that the ABS module works OK on his EV conversion FL1, so it's able to ignore the fact that there's no actual engine fitted. However looking at how well the EV spins the wheels, the ABS module isn't able to initiate TC, without knowing the throttle position.