Speedometer solutions

[CWXC70]

OK, so I have fitted a G80 into my 1031 axle and have come across an obvious problem and one that on the face of it should have a simple solution.

Mine had an electronic speedometer which took it's signal from a 12 gap tone ring on the diff, sensor in the steel diff cover.
The G80 has a 48 gap tone ring which must also do the ABS in the 940 it was born in. Great, cut every three teeth out and you have 12 teeth and 12 gaps.
I've carried over the 940 aluminium diff cover with the 940 sensor and instead of getting a 75% reduction in pulses and a correctly reading speedo I'm reading high, probably not 4 times as high but with a distinct rise and fall on the needle at low speeds.
a) Does anyone know how different the sensors are, i.e. they both appear to be two wire variable magnetic reluctance sensors and not hall effect (three wire) but is there something special about the 940/ 48 tooth type that I'm missing

I'm a bit foxed as to why the reading is high as I am missing a load of teeth unless it is picking up the background steel and not getting a nice clipped signal. The 12 tooth tone ring doesn't have a diff right behind it

b) anyone have any other great work arounds before I start adding 12 tooth wheels on the prop?

[Clifford Pope]

As you now have the number of teeth approriate for a 240, why not use the 240 cover and original sensor?

[CWXC70]

Cheers, I will try this, hopefully the depth that the two sensors protrude in is the same. My concern now is that the sensor was reading too much diff and not enough air.

Really kicking myself for cutting up the 48 tooth tone ring now. That was at least reading 4 times too high nice and cleanly!