hmm,, they are pretty simple..
look first divide the number of teeth in the rear sprocket by the number of teeth in the front sprocket,,u get a number called a sprocket ratio,, what this ratio means is that the front sprocket must be rotated that number of times to get the rear sprocket (or wheel for that matter ) to rotate once..
for 37\15 sprocket combination the ratio is 2.46 , i.e front sprocet must rotate 2.46 times to rotate the rear sprocket once,,
if this number is 1 it means that every 1 rotaion of front sprocket rotates the rear sprocket once and that will require a good torque ( another measurement scale ,which means ka kitni taqat kisi cheez ko gumanay,ya rotate karna ma lagti hay) ,
now the stock ratios are always meant to offer a good balance between the speed and torque,,at all gears,,
so if u jack the ratio higher the speed will reduce,,because more turns are needed for the front sprocket to rotate the rear once,,
but u will get more torque then what u were getting earlier at same rpm,,
so with more torque u will be able to quicky accelerate and also u will be able to climb high slopes,, yani pahlay agar double sawari pa bike aik charhai nai char raha that,,ab usi ger and throttl level pa nisbatan asani sa charha ga,,
the more u jack the ratio the more it will be easy tro climb,,
so if u intend to use ur bikes on such planes u have to increase the ratio,, either by playinbg woith different sprockets ,, ya tho agli ais lagavo ka ratio char jaya ya pichli ais lagavo ka ratio char jaya,,
normally most people who play woith sprockets go 3 teeth up on rear and 1 teeth down on rear,,,
now if u reduce the ratio then the stock u will get more speed but ur torque (pick) will reduce,, s u may get at top gear top throtle single sawari a good 100km/hr speed but on double sawari u will hardly get 90,,, because the torque is reduced so are the rpm and hence the overall speed,,
if one intends to always use the bike in plain areas the reducing the ratio will be good thing as it will give better top speed and good economy,,
there is a lot lot lot mre to this sprocket ratios then just this,, but hope it will now feel more natural to u,,,