Odometer

[psdayama]

mmm another idea here, as you already have something workable, you can probably modify it. Is this a Cable system? If so, you can probably modify the dash end and attach a Optical Encoder to create your VSS signal. Optical encoder OR Using some optocoupler stuff.

I think U cant make odometer with just Hall sensor. What U need is Quadrature optical unit. By this U can sense whether car is going forward or reverse!!
Unless U want the reverse distance travelled also added. With quadrature opto unit and 2 JK flipflops U can generate + & - ve pulses per
revolution and then just count them and divide by pulses/mile or Km whatever
is Ur unit. Also then U can calculate the speed also to be alternately displayed. All cumulative distance should be written to EEPROM when switching off occurs and read again after next switch off.
I leave it to masters for genarating code on this line. I have serviced
such Odometer used in runway painting. It was based on Z80.