I searched the forums for the information I needed in this regard. Now I want to throw out an opinion. I took my plated 07 WR450 in for its first 20 hour service to check valves, etc. It was also time to put on a new rear tire. The original was already worn out. This bike spits out tires like nothing. It is one powerful machine. I have done all the free mods and added Q4 pipe, baja kit, jetted etc I had the dealer put on a new Michelen Enduro 140/80 tire. I wanted to experiment vs. stock size. Mainly, I wanted more traction...a little more hook up. I currently run 13X50. I took the bike out today and hit the technical single track, hills, and rocks in Salt Lake. It is very noticable that with the larger tire, the bike is faster, and I have to go to a lower RPM in the tight stuff, and feather the clutch more. This new tire really hooks up. Now, when I hit the gas from a slow idle in the tough stuff, the front end comes up more (I am sitting more towards/on the tank more!). I want to keep my RPM's up a little more in the tough stuff. I was thinking of going to 13X52 to compensate for the larger tire. I know I loose a little top end, but frankly that does not matter to me. I am in first quite a bit, and would not mind using 2nd and 3rd more (beside when I am in the more open riding). What do you think, 52" in the rear? I want to keep the front the same. I think that would make this bike perfect as I love the traction and the way it climbs with the 140/80. Thanks for your input...good or bad.