the 15" wheel is a huge mod, the same as mine,,
it gears it down by heaps, some 5 or 600 rpm for taking the load off the clutch on loaded take offs, makes it rev a bit higher at 100klm but you get that
they machine the beads and spokes down so a 15 car rim fits over whats left, if you look real close you can see blocks between the spoke ends which is used to bolt the 2 together,
the bolts run down the length of the spoke thru the rim