A JD 550 is a 60" tiller from what I can tell. That is no problem for the CK30, it's what I was going to get when I had my CK30. Here's a post from a JD user with a 955. That is a 33hp tractor and I've owned one of them too. It runs around 3300 RPM to get the 33hp and really should be rated closer to 28-30hp. Having owned both, the CK30 is stronger. Check out post #2.