BruceP

Apologies if someone already covered this and I missed it when I scanned the thread, but in case not, here's the relationship:

That first number in a tire size is supposed to be its width in millimeters. The second number is its aspect ratio, which is the height of the sidewall expressed as a percentage of that width. So rubbing is possible in two ways: the tire will be wider (that's for sure, even if it's not by the full 30mm), and the tire will be a little taller if the aspect ratio wasn't reduced in proportion to the new width.

It's true that a too-narrow wheel will reduce the contact patch, but it does it by rounding the profile of the tire. It doesn't narrow it per se. What it does do, though, is negate some of the advantage of the wider rubber.

Dunno if that's helpful.

Shark Attack

Im not trying to be an *** here.. But talk to the local tire company.

My experiance is you can get pretty wide on the original rims... My rears are way wider than stock and they are on my stock wheels

BruceP

Without what happening, exactly? Can you mount wider tires than recommended? Sure. Will they explode? No. Will they look great? Probably. Will they hurt performance or safety under 'normal' driving conditions? Probably not. Will they put rubber on the ground in a useful way from a performance point of view? After a certain point, no. At a certain point, it becomes a matter of cosmetics over performance, and your tires will eventually tell you so by wearing unevenly. There are lots of local tire companies that would say something like this is "fine," without bothering to ask what the owner expects from the car. Given it's Dell we're talking about (or a friend of his, anyway), I thought the technically correct answer was appropriate.

MiamiC70

I have 295's on OEM rims on the rear of my 2004 and fit fine. Lots of room and no rubbing anywhere.

Sneaky Pete

Summer for me are 295"s with the X74 (1 3/4" lower than stock height) and no rubbing. Plenty of room.

LVDell

Kyle, you are being an *** and obviously didn't read my first post. Having been heavily involved in DE and racing for years, don't you think I understand so simple as that? I never asked for that type of help. Again, re-read my OP.

LVDell

Thank you everybody for your responses.

I fully understand tire sizing.
I fully understand profile/series in relation to sidewall
I fully understand that each manufacturer publishes their own specs
I fully understand that two identical sized tires from different manufacturer can differ in actual size (see previous statement).
I fully understand there is an optimal wheel size application for each and every tire
What I do NOT fully understand is how exactly tire width is affected when moving up a size (or down on the same wheel). I don't believe it is linear which is why I asked.

Many of y'all helped explain that very well and I appreciate it. I feel much more educated on something and I have y'all to thank

BruceP

Geez, scientists are cranky.

Since your last line is still in the present tense, I'll reframe my answer to be more specific to the question.

It will increase, theoretically in direct proportion.

Tire widths are nominal and vary in truth from one tire design to another, like trouser waistbands, but they are meant to be read as linear measurements. Also, the widest part of the tire is not necessarily at the tread. In theory, his worry is valid. It could happen.

If by contact patch you mean how much tread touches the road (versus how well distributed pressure is), this is irrelevant. The contact patch occurs where the tire is deformed by the weight of the car. It's at the pavement, where rubbing isn't a concern. As for the overall width of the tire, this should change very little by being put on a narrower rim. The rim is just altering the shape of the profile. The risk of rubbing will increase further if the wider tire doesn't also have a lower aspect ratio. Thus the risk of rubbing is not only attached to width change.

Bottom line: everybody can offer lots of input, but not much help. The only ways to know are to look at another car with the identical setup, including the same make and model of tire, or to try it on his own. There are too many variables to give him a reliable answer in the abstract.

LVDell

Cranky when I haven't had my coffee and posts like Kyle's are submitted. Ao I had to be clear what I know and don't know

Bruce, you I like

outline

LVdell,

no worthwhile change in tire width on the road surface, just sidewall angle change (inwards or outwards) up to a point upon where the sidewall will not hold or fit on the rim. it seems the sidewall limit will be reached before theres any real impact on the shoulder at the tread, whereby it could start adding extra width or indeed reducing it.

the rolling surface is hard, the sidewalls flex so all you do is get a pivot point at each edge where the sidewall meets the tread. there will be some small radius change as the sidewall stretches outwards or inwards but its not impacting the tread width measurement by anything significant.

to gain from a wider rim you need to use a wider tire too.
now theres additional issues over how the angle of the sidewall impacts flex and feel but i fall asleep by this point.