Made some tests with 1.1.50 and 1.1.53 Non-PS2 firmware.

Hardware on PS2/CLK pin:

* 4k7 pullup to +5V and 5v1 Zener regulator to GND (inside ECU case)

* series diode points to stock speed signal switch in differential. It switches GND to a +12V-pulled signal that drives the speedometer nicely

* 9 pulses per wheel rotation (around 122Hz at 100km/h)

Signal with connected as above:

Signal with a 1uF capacitor to GND

(it seems too large for higher frequencies)

I've made tests with no, 100nF, 200nF, 500nF and 1uF capacitors and adjustments of edge and minimal period time. It didn't really change interpreted signal quality.

At lower speeds the signal is all right, but at higher speeds it becomes very noisy

more logs here: http://quasar.dynaweb.hu/~lezsi/vems/wheelspeed.zip

I don't have a 6 speed gearbox, but have a long diff. ratio so calibration data of gear 5 (233) and 6 (255) is close enough for the routine to make bad guesses. (This range is tight anyway, strong cars go faster than 254km/h at 5000rpm)