What he said.
Also, will low gears like that and 18" tires, you shift to much and you are barely in the power. It's amazing what that taller gearing will get you that blue was talking about. If you need a little more power to push the taller gearing, get some compression. For mx 180 - 210 psi isn't uncommon. Just need to run good race fuel if you go that high. I agree 100% you need taller gearing.