Thanks for your help so far but this seems more complicated than I would have thought. I think I asked the question in a wrong way.
The first number, currently 225, is the width of the tyre.
The second number, currently 70 is the ratio of width to sidewall hight.
Third number, not changeable, 16 is the rim's diameter.
My question being:
What is the maximum width of the tyre I can put on my rim? How do I find out what width my rim has? I presume this is the only real hindrance when it comes to putting 31'' on my stock rim. Also, even if my rim would be able to safely hold wider tyres, wouldnt they start rubbing because of the rim's stock backspacing?
EDIT: Apparently someone on the forum argued this: "The 16" wheels were 7" wide, and if I'm not mistaken, they were only available as an option on the Sahara (upgrade from the standard 15" wheel that came with the package) through maybe 2002 at the latest. They should be a silver painted 5-spoke wheel with relatively slender spokes..."
If someone tells me how, I will check this. So now the question is: How wide can my tyres be so that they fit my 7'' wide rim?