Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator

 

I notice you have knobbies on your KLX. If you go to 15/42 you may have real problems in some technical or steep terrain, 351 or not. The buddy I bought my '06 KLX from had the 15/42 gearing with some Pirelli street tires, and even then it was a slug in first gear...albeit in 250cc size. That's some high gearing for anything off road. If you're going to do mainly pavement or just dirt roads, it will probably be fine with a 351.

 

I did a little single track woods this weekend with the 15/42. The gearing is great on the highway, much more relaxed and the 331 chugs right along. Single track was fun, I kept trying to downshift until I realized I was in first. The gearing was a bit tall, seemed happy at about 12 to 15 mph which was at times a bit much for the trails I was riding(tight woods). Am glad I have 13/14/15 front sprockets to make the bike cover all the bases.