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??

[Evangelion]

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.

[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?

[HustleRussell]

There's a very useful interactive calculator here which you can use to calculate and compare tyre circumferences:

http://www.alloywheels.com/Tyre_Calculator

Personally I'd go to a 185/45r15 or possibly 175/50r15 (which matches the circumference of the 195/45r15 perfectly).

It would certainly be possible to fit the 195 onto a 6" rim but the tyre would be pinched inwards at the beads- whether this would cause a perceptible difference to handling I am not sure, but I wouldn't do it personally.

[AtticusFinch]

A question. If you fit wider wheels and tyres should you keep the tyre pressure the same?

Logic would suggest you should run a lower pressure but somewhere I thought I heard {in the dim and distant past} you should runa higher pressure in a wider wheel/tyre set up?

[stevieturbo]

What does the tyre manufacturer recommend ?

Straying too far either side probably isnt a great idea. But then look at all the dheads who fit narrow tyres onto wide rims to achieve a stretch. As much as I think it is ridiculous, there havent been too many serious accidents because of it. Which in itself is amazing.

[AtticusFinch]

No recommendations. It's a "67" Sunbeam Alpine. It now has 185/70/13 original tyre 6" x 13" @ 24 PSI

[SlimJim16v]

A 195/45 on a 9" wheel is crazy. 6" with 7" max I'd say, so 6" will be fine.
Unless you want the Euro look, with narrow tyres dangerously stretched onto a too wide wheel, yuck.

[littleredrooster]

SlimJim16v said:

A 195/45 on a 9" wheel is crazy.

Sorry, but where did the 9" wheel come from? I thought we were talking about a 6" wheel...

[SlimJim16v]

Meant to say 8", as per the OP "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"

[PaulKemp]

195 on a 6" is fine 1/2" less wheel width equates to 1/4" on each sidewall no big deal, but you are near the limits for correct handling, I would question the low profile on a classic car, most need a bit of sidewall to help the suspension.
We run 205-50-15 on 6" no problem
I am moving from 185-60-14 to 205-60-13 on the rear and find its a bit wide for the rims (6"), will change to 6.5 or 185-60-13 when I have worn out the 205 R888's