From sifting through the forum topics over the last month, I learned that most people recommend running 20/ 39 for the best overall performance. SO I wanted to see what was in mine, It's a 96 xcr 600 with the 680 ultra spx motor in it. I figured it would be a 21/ 39 or somthing like that but here its a 21/ 35.. Never heard of that combination. Anyone run it or know anything about it, just wondering. I'll be sticking the 20/39 set up on it anyway..