To avoid confusion, Plunger bikes all use the same primary transmission, engine/clutch ratio cannot be changed with standard parts, so this is fixed. Gearbox and rear sprocket can be mixed'matched, big gearbox sprocket with small rear gives relaxed high speed cruising, but poor acceleration. Conversely small gearbox sprocket, big rear give good acceleration, but motor revs its socks off at higher speeds. Latter low gearing set up is used for sidecar work. So for specific functions the ratios of the final drive can be chosen to suit, within reason.
Bear in mind that in top gear the gearbox mainshaft and output shaft are locked and turn at the same speed. Considering the later S/A models, the same applies to the final drive. But although the S/A clutch chainwheel is the same for all models, the primary crank sprocket can be changed. The above parts list notes several variants, this is because the sprocket and drive sleeve were changed early on, and a mis match brings up all sorts of chain alignment issues, well known on the Forum.
A smaller crank sprocket lowers the overall gear ratio, in other words the engine must rotate more times for a single turn of the back wheel. For a given engine speed the road speed will be lower. Conversely, a bigger crank sprocket gives higher road speed for a given engine speed, or same road speed, at less revs. Here again, it's all down to riding style and use. Highly geared is fine for high speed work, but less acceleration, and useless for load lugging or slogging up big hills, here the motor will not be able to reach the sweet spot in the power band without constant use of the lower gears at the slightest incline. This is often referred to as being "overgeared"
The question of ease of starting is down to the larger diameter of the bigger crank sprocket. Simply put there is more leverage applied to the crank. The downside is that it won't turn as quickly or rotate so many times with a single kick.
Speedo drive ratio is affected by final drive changes. Primary ratio change has no effect, but road speed per 1000 revs will change.
Swarfy