This Indian Standard which is identical with ISO 1977:2000 'Conveyor chains, attachments and sprockets' issued by the International Organization for Standardization (ISO) was adopted by the Bureau of Indian Standards on the recommendations of the Continuous Bulk Conveying, Elevating, Hoisting, Arial Ropeways. 
earing down by installing a larger rear sprocket (like with our YZ example) increases the final drive ratio and reduces top speed, but can increase acceleration. Gearing up, like with a smaller rear sprocket, decreases the final drive ratio and adds more top speed to your motorcycle or ATV.
Standard quality plain sprocket 40teeth 1/2pitch
Price depends on quantity