it gives the same ratio as 18/48 I think
Might have been used to lower weight.
a 48 tooth 630 would be roughly 11.5 inches across, and a 40 tooth would only be about 9.5 or so inches I think, a good chunk of metal lost that you wouldn't have to accelerate
If someone went through and did serious work on the motor it would make sense for them to carry through elsewhere on the bike.
math may be off, not mathematicaly inclined here.
630 = 3/4 inch, 40 x .75 30 / by pi gives 9.54 dia.
* 48 yeilds 11.48
ken.
