Tyre width allowances on wheels

[Adamal]

So we have 2 things here. Wheels, width measured in inches, and tyres, width measured in millimetres.

Now you don't seem to get clean measurements interacting with one another. For example, the standard wheel width for the front of an SW20 is 6", and the tyre width is 195mm.
Now, using the nifty converter on my phone, we find that 195mm is 7.67" and that 6" is 152.4mm.

What I want to know is this: Is there a practical allowance of tyre width over wheel width? If you get my meaning, as it seems in most applications (Bar Bosozoku style!) that the tyre is slightly wider than the wheel.

To be even more specific, what would be the widest tyre you'd put on a 9" wheel?

Thanks for any help guys!

[Lloyd]

Add an inch and a half onto what your rim size is maybe? I've got 215s (8.46 inch wide) on a 7 inch rim which is about right. Would mean that for a 9 inch rim you'd be looking at 265s, possibly 275 if you cant find 265s.