There is a significant amount of debate on the issue of running floatation size tires on Mitsubishi's narrow 6 inch factory aluminum and steel wheels.
Whether or not this practice is wise is something each individual must decide for themselves. However, there is a reason why a tire shop might wish to decline mounting tires larger than 30x9.5's on 6" wheels for street use, and that reason is bead seat angle.
In low traction conditions, such as off roading, there is perhaps less of a likelyhood of bead seat failure with 31x10.5's on 6" wheels than there is in high traction stiuations, i.e. pavement use.
A vehicle negotiating a turn on pavement places a significant side load on the sidewall and bead area of a tire. This is one reason why sportscar enthusiasts use low profile tires mounted on rims that are typically equal to the tire's TREAD WIDTH. Doing so enables the tire to plant a full contact patch on the road, without undesirable flexation in the sidewall, and without the sidewall trying to "roll under" the wheel. This can and cetrtainly does happen on tire/wheel combos where the rim width is narrower than the tread width , and is magnified with the high aspect ratios used on off road flotation tires.
Personally, I don't like a lot of sidewall bulge in tires in general, and certainly not in tire/wheel combos that I use where I do most of my off road driving. Sidewall bulge in the terrain that I traverse could cause the sidewals to be damaged either visibly, i.e. cuts from rocks, or bruised. Some bulge is desired, though, to protect the wheel. I would say that where a 31x10.5 is concerned, an eight inch wheel width will still provide adequate bulge of the tire for rim protection.
Not all tire beads are created equal, too. Some are stiffer and more resistant to separation from the rim than others, and some can tolerate more angularity in mounting than others, too.
The previous owner of my 88 Montero had mounted 30x9.50's on my sport's 6"wide cast aluminum wheels. With a full load of camping gear, I would get tire rubbing where the inner sidewall would contact the frame. I now run 31x10.5's on 7" wide aftermarket wheels with no rubbing.
When considering the switch to larger rubber, one should ask if the bigger tires will enable them to get where they want to go better than a smaller size. Put another way, there is a significant difference in weight between a 31x10.5 and a 33x12.5 tire... sometimes as much as fourty pounds ! Extra unsprung weight will make every component in your drivetrain work harder. Your clutch, wheel bearings, CV joints, U joints, shocks and brake components will all be stressed more as tire size increases, and with the crummy 1.98 to 1 "low" range of the Montero, taller really might not be the way to go.
Another thing people assume is that big tires mean more traction. Ain't necessarily so. Ground pressure plays a big role here. I believe that it is possible to have too much floatation, especially when climbing steep grades of loose rock, for example. On a fairly light vehicle, such as a 2dr Monty/Raider, a 31x10.5 ought to be a good compromise.
The point is really simple: A tire/wheel combo that many would consider unsafe on the street may work fine in the dirt, and just because somebody has been lucky in defying the laws of physics, one should not assume that one's luck will be as good as another's.
Stick to the tire manufacture's suggestions for wheel width with their tires. After all, who knows more about their performance? The people who designed and built them, or some geek on a computer who uses them on weekends, and maybe not in terrain as challenging as what you wheel in on your stock 235/75 15's ?
Food for thought... From a guy who can remeber the good old days when a 10-15 Gates Commando was a big mutha of a tire, and we did the Rubicon on them without major incident related to tire size.
Ask yourself honestly if the wheeling that you do is as tough as, say , a Camel Trophy... where those Land Rovers used skinny tires and open diffs and made the trip.
[ 16 December 2001: Message edited by: PHIL ]