Different rules about tyre and wheel widths

[alolympic]

I am trying to decide on the size of tyres needed for my car.
Car is a modified classic, so no original spec to work from.

I have been reading about which width of tyre to fit to which width of wheel, and I am surprised to have come across a new guideline.

you can find it written here amongst other places -
http://www.carbibles.com/tyre_bible_pg4.html

For my example, you can choose a 195/50 15 tyre - it says you can fit to a wheel between 5.5" and 7" wide

If you then just reduce the aspect ratio to a 45 - so 195/45 15 - it says you then need a wheel between 6.5" and 8" wide.

Problem is, my wheel is 6" wide and the 195/45 gives me the rolling diameter I want.
How seriously should I take this guide?
What will the result when fitting a 195/45 to a 6" rim??

[alolympic]

Evangelion said:

The lower the profile of the tyre, the nearer its width has to be to the width of the wheel, because the sidewall can stretch less to accommodate the difference.

Brilliantly put, thanks.
So, would my tyre choice on a 6" wheel be undesirable then? would choosing a tyre with a relatively soft sidewall, like a Continental Premium Contact lessen the impact?