I have been running 55-60 the last 24,000 miles or 7 years with 15" tires, but the engine started making loud noises and I took it in for a rebuid or whatever it needed to a dependable shop I know of. They did just the top part of the engine as the lower part looked recently rebuilt. (Now I'm not engine smart so exuse my wording), they had the head machined etc. They told me to take it easy not over 50mph, so I thought I was taking it easy, but it got real loud and knocking so I took it back and the valves guides??( I think they said) were all out of whack if that makes sense. They said I need to do something with the rear end gear, so thats where I'm at. Does this make sense? Thanks!