I have a 99 450ES and it seems as though 40 MPH is the fastest I can go. Mine is a bone stock engine on 26" Mud lites. I will soon be installing the 4-2-4 in hopes that with the front diff disengaged i will gain a little speed so I can keep up with the others I ride with. One thing I will say is that the 450 has some power. I may not always keep up but they sure like it when I can pull em out of the mud. I had to pull my buddy's Polaris 700 twin out 3 times today. Makes me really appreciate what these were designed for.