I started with MTRs and bought a set of IROKs to move up to the 39.5s. As everyone seems to be in agreement (MTRs for street & trail; IROKs for trail only) the only comments that I would make in addition is you really need to air down the IROK to break it in and for it stick in the rocks. The other possible issue is the sidewalls on the bias ply IROK seem to be thinner than the radial MTR. That being said, I haven't had any trouble so far with the IROKs and at 4psi they do hook up.