AP50 Cruise Control and Speedo Sensor

Replying to a post in January (postID:9702) "joc45" and "chrisfrd" had some very helpful info on using the existing speedo sensor to drive the cruise control instead of having to hook up magnets to the tailshaft. As a retired clerk I now have time to fiddle and am enjoying it immensely but don’t have much experience. I am fitting the AP50 to a 100 series Standard wagon and the speedo sensor in the rear of the transfer case has three wires: one grounded, one to the ignition and one to the speedo. From a remark by chrisfrd I take it that the two ungrounded wires are the ones to connect to the AP50. I don’t know how the sensor works but my query arises from the fact that the schematic circuit diagram in the Toyota manual shows a connection inside the sensor of the +12v via a resistance to the wire to the speedo. I know the diagram is only a schematic but how does this sensor provide a voltage in the order of 0.8v which is what the AP50 manual suggests would be present from the magnets on the tailshaft at a speed of 50kph?
On a different issue, the current AP50 unit gives an option of cutting an external black wire loop in order to change the range of the unit from 2000 ppm (pulses per mile) to 5000 ppm. Choosing the latter means 1 pulse each 0.32 metre or 43 pulses/sec at 50 kph. The Toyota has 30.7inch tyre diameter and 4.3:1 diff so 2 magnets on the tailshaft would result in 49 pulses per sec at 50kph. Is this right and therefore do I cut the loop?