There's something I don't understand about altering the gearing and people say, oh yes it gets to 60 much quicker or it does more miles to a tankful etc, how do they know?
I think the speedo might be off the gearbox, because in the reflash tools there's options to adjust the speedo for
sprocket changes for this reason.
In terms of 'how do they know', because it's a long established practice and you can calculate it all from simple ratios.
Also, simply think about riding a pushbike, as their gears are doing exactly what we're thinking about here. For any given front
sprocket size, when the chain is on the largest rear
sprocket, it's much easier to pedal, but you don't get a fast speed, so you then drop to the next size
sprocket, which takes you faster but gets harder to pedal. Each
sprocket reduction is increasing the number of wheel revolutions for each rotation of the pedals, but reducing torque at the same rate.
Put enormous
sprocket on the front and tiny on the back, and you can cycle at crazy speeds like Guy Martin did behind the lorry, but of course, because of the huge drop in torque now produced at the rear wheel, he needed towing up to a speed where he could then manage to continue and needed the lorry to remove air resistance as he wouldn't have the strength to pedal against it.
Since the
sprockets on a motorbike are the final drive, you select the ratio that is best suited to the riding you do. The bike manufacturers get this pretty spot on IMO, but people sometimes like small adjustments.