I am looking to gear down my 07 xr 650 l. I want to go as low as i can. will sprockets for a 650 R work on my L? there are alot more choices for the R. I was thinking of going 13 front and 54 rear. Is there a mathmatical formula for figureing out the speed of this switch? i got stock wheels. I am not worried about the top speed i am using the bike in dirt only. 60 mph is fast enough through the woods where i ride. most of the time i am between 20 and 40 mph.