The original 150’s ran 15/43
with 19″ wheels
The standard gearing on later models changed to 15/40 with the 19″ wheels,
some Foggys have 17″wheels , somebody mentioned a minimum of 38teeth on the rear (can’t rember who !).
My six is running a mitus 130 on the rear instead of the 140 , for a better tyre profile , with its smaller circumstance I changed the gearing to 16/40 for mph16.126 per 1k rpm , a tiny increase over standard
(The front sprocket chain guide only fits 15t) take a look through the screens on the Speedo (saved maximums) to see what your mph / rpm
works out at .