Impulse/magnetic speedo issue

[Oldbones]

So… Have a BMWR1200c that was triked.
Have now refurbished the propshaft, rear axle (Reliant Hi ratio) & fitted disc brake conversion.
The original creator (unknown) has used the real wheel sensor and mounted it over the prop rear coupling. However we are seeing nearly double the correct speed!
Which will also incorrectly record mileage etc.
Any clues as to what’s wrong, or who can assist.
Can use gps speedo on phone, so avoid getting nicked for speeding, but hate stuff that don’t work right?
Cheers Oldbones

[Aztec]

So… Have a BMWR1200c that was triked.
Have now refurbished the propshaft, rear axle (Reliant Hi ratio) & fitted disc brake conversion.
The original creator (unknown) has used the real wheel sensor and mounted it over the prop rear coupling. However we are seeing nearly double the correct speed! As the prop shaft spins faster than the wheels you would expect that
Which will also incorrectly record mileage etc.
Any clues as to what’s wrong, or who can assist.
Can use gps speedo on phone, so avoid getting nicked for speeding, but hate stuff that don’t work right?
Cheers Oldbones

Any chance of a couple of photos to help our experts out ?

[Skidmark]

when I set up my Acewell I mounted the magnet to the rear brake drum, then I set the speedo to the distance of one revolution of the rear wheel, I marked the ground and the tyre rolled it along the garage floor and marked it where the mark on the tyre met the floor and measured between the two marks and set the speedo to that distance, you will need to do the same but as its the prop shaft that its measuring the one full revolution of the prop and measure the distance travelled by the tyre.