Someone else on another forum said this in relation to a 99 ranger.
Well from my experience 31x10.5/15 will fit the stock rims, but under hard driving may rub. They are just a bit too wide, but only just a bit. 32's are pushing it on the stock rims, and will rub much more often. But if you get some negatively offset rims to replace those stock ones, you can go up to 33's without to much problem (as long as they are under 12.5" wide). But anything more will need a lift kit of some sort, or some fender work.
Hope that helps.