In my opinion and experience, 15/41 is too high for MX. Just my thoughts and 6 years of racing. But - I run 14/38 (or 15/41 same thing) with 18" tires. For stock, or 20" tires, stock gearing is better (14/41). Depending on traction of the launch, I like to start in 2nd gear. With this high a ratio, this could not be done effectively with 20" tires. Some people prefer different set-ups but if you can launch good in 2nd, you will have a great shot at the holeshot. When all the other guys are shifting to 2nd, you are still on the power and flying by them.