The tyre width suitable for a rim is primarily dependent on the use the wheel is going to be put to.
With a 5.5" rim a 235mm tyre (9.25") will be 1.9" wider at the grip surface than at the rim. When climbing out of ruts or other activities resulting in high forces on the edge of the tyre you're extraordinarily likely to pop the tyre off the rim.
This tendency is exaggerated further by running tyres at lower pressure to increase grip.
Do bear in mind that wide tyres are not actually that beneficial in mud. The tread pattern is much more important and a narrow tyre will cut through the loose surface and grip on the stickier stuff underneath. A wide tyre will float on the surface of the mud and cover everyone behind with crud but not get very far.
Ideally for mudding you want narrower tyres of a high diameter to resist the tendency to dig a hole.
What are you going to use them for?
