i agree with that, good for road readings but not off-road.

What i did, as i didnt want to drill the spur gear. I put the magnet on the front drive shaft. You have that problem as where you are putting it, it has a large cetrifugal force due to the circumferance. Basically it has to travel faster to make one rotation of the centre compared to something with a smaller circumferance, even though they have the same rpm

all i use is elecrical tape and it hasnt fallen off since i got it. I wrapped it up nice and tight so the bulge of the magnet protrudes almost streaching the tape

This theory is probably not exactley right, but its true and works.