A 140/80/18 will last a bit longer and is even a bit more bad ass and good hook up.
I put a 140 M5B on my WR450F about a month ago.
I haven't ridden in really slick mud with it yet, but yes it hooks up great in anything that is loose, including the bit of mud that I did ride in.
The front edge of the knobs do round off fast if you are riding in rocks and are on the throttle a lot. Some people swear by the Michelin S12, saying it has similar bite and somehow the knobs stay square on it. I haven't tried it yet.
The 140 M5B does rub on the inside of my FMF muffler though. I'll have to put a spacer between the muffler mount and the sub frame to give the tire a bit more clearance.
I'm not sure a WR450F needs a 140, especially a tire like the M5B. On the other hand, I've "cleared" every hill I've tried to climb since I mounted it. By cleared, I mean that I didn't have to get off and push. I may have had to restart a hill, but that is due to pilot skill deficit, not bike/tire issues. In contrast, I frequently lost traction with the stock MX51 rear tire.
Edited by MidlifeCrisisGuy, August 05, 2014 - 09:08 PM.