How does this work then?

Ask your local tyre firm. THey should have a chart.

Lets put it this way though, applying a little common, if the rim is more than 10mm wider than the tread then the widest point on the wheel will always be the rim width.

"Rule of thumb" is tread is effectively 5mm wider (or narrower) per

Nominal rim width should be about 80% of tyre section for modern lower aspect 70/60/50% tyres (70% on old cars, trucks, vans, 4x4, suv's etc with 82% tyre profile).

Nominal tread width - nominal rim.

The 165/65 should be on a 5J and will be slightly "stretched" thus giving a contact patch like a 170. The 185/60 should be on a 6J and will have a contact patch like a narrower 180.

Very low profile tyres need wider rims, for 45% the ratio is 85% and for 40% it is over 90%.

So for your 195/45 the nominal rim is 195*0.85 = 165.75 = 6.5". On the 6J it will be about 5mm narrower and on a 7J 5mm wider, 190 and

Well you would think so, but applying common sense without a bit of research is a great way to cause big problems later on.

Perfect! Thanks.

That's just the thing I need.

The "issue" with such dinky arches and poor clearance is that fitting suitable alloys in the space isn't easy.

Oh and keeping low wheel weight too.

If I can find some TSW Quads in the right size, great, but from the above the difference between a 6J 15" wearing 195/45s or a 7J rim with the same tyres isn't great. Still, 10 mm may be enough to put it over the edge... :-(

OZ Superleggeras are available _from_ 7J 15". :-/ They're about the most stylish, lightweight wheel we can find.

In that case then, you need to be paying as much attention to the offset as the wheelrim/tyre widths.

The offset bit is covered, but what comes first, the offset or the effective width of the tyre. :)

Offset, naturally...