Hi Griffo
Just having a look at this for myself at the moment as I just bought a C15S, looks like there are a few options to up the final drive ratio.
1st option and the easiest/cheapest would be to see what’s on the final drive front sprocket, sprockets available are 13, 14, 15, 16, 17, 18 and 19 teeth, and the standard rear is 45t (final drive ratio: divide the driven by the driver) Highest with a 19t front (45 / 19 = 2.36:1) Lowest with a 13t front (45 / 13 = 3.46:1)
2nd option is the primary drive, can’t see as many primary duplex drive sprocket size options but the documentation I have 18t for the engine and 52t on the clutch sprocket as std. However TMS Nottingham are showing a 23t engine sprocket, you may need a new primary drive chain to fit that though.
Highest with a 23t primary drive sprocket (52 / 23 = 2.26:1) Lowest with 18t primary drive sprocket (52 / 18 = 2.88:1)
The final option is the gearbox, looking through the documentation I have there are 3 ratio options for my build year (1962) Changing the gearbox would be extreme, I will be trying the first option, although I am also looking for a spare engine as a donor, depends what I can pick up for the right price.
Attached is the PDF from BSA showing all the ratio options.