I have a highly modded 300ex motor and when i bought the bike it had a 15 tooth countershaft sprocket and a stock 38 tooth rear sprocket. I went back to a 13 front becasue i wanted the chain gaurd. What size rear sprocket should i run in order to make the bike run like a 15,38 setup. I thought a 13,36 tooth setup would work. But then an Atv mechanic told me that i should be running 6 teeth sizes down on the rear to get the 15,38 setup. He said for every tooth up on the countershaft to go down 3 teeth in the rear.
So i was going to put a 33 rear sprocket on the bike but now i am confused. 33 is the smallest sprocket they make for a 300ex/250x so i cant use a 32 tooth sprocket. Thankx in advance

 

A 15 tooth front sprocket fits in there?? I suggest going to sidewindersprockets.com they have a sprocket ratio chart so you can see what is close to what you need. They can make you any size sprocket you need I think. With a 13,33 sprocket combo the ratio is .01 more than with a 15,38 sprocket combo so I think the 33 tooth would work good.

 

just do the math:
15/38=.394
13/.394=32.99=33 tooth sprocket

the 3 tooth on the back equals one on the from is a good rule of thumb