What does moving up in tire size on same wheel do to tire width?

Have somebody that asked me this question and I honestly don't know the proper answer. Here's the scenario. Wheel is a 10" wheel with a 265. Guy wants to run a 295 on the same wheel. Without getting into what is the proper size wheel, etc, what will happen to the width of the tire? Reason I ask is he is worried that increasing the tire size will increase the width by the the 30mm difference in tire size (15mm to each side) and thus will get rubbing. I can't imagine it's a true 30mm wide on the same wheel since the sidewall and contact patch is affected by the wheel width and thus stretches or bunches the tire. Any input would be greatly appreciated.

What car is he planning on running them on? An older gen 996 might have fitment issues without an offset change.

Wheel width and offset will stay constant. Just wants to move to a wider tire. I thought I remember reading that moving to a larger tire doesn't necessarily mean gaining a larger contact patch as the tire width is affected by the wheel width.

I think the other issue is the profile of the tire. I assume your friend would drop from a 35 series tire down to a 30 series tire, if memory serves me correctly. I'm not sure on the width. Also - wouldn't it look a little silly putting that tire on a 10" rim? I think it might bulge out a little bit over the edges of the rim.

the 295 will fit on a 10"... It will be close on the inside lower front near the subframe rail... I run a 285 Hoosier on the rear with a 10" wheel and it it REALLY close.

There is no formula to estimate the actual contact patch or tire width based on the rim width. It is almost impossible to predict as tires vary in manufacturer, tire type, stiffness and measurement. Even if you use the same manufacturer, same tire type, the tire measurement can still be off as some tires will be a "big 245" or a "narrow 285". You can reasonably expect the tire width to be within 5-10 mm of the stated width, or that the increase in width from 265-295 will be around 25-35 mm. 295 tire with the right sidewall should fit any standard NB or WB 996 on an OEM like offset. I'm running a soft round shoulder winter 295 on an aggressive 11" et 51 wheel without prob on my NB 996. Tire rack is very good at knowing the subtleties of tires and can tell you if the 295 is a true 295, whether the shoulder is round or square, and whether the sidewall is stiff or not. Plus, if they say it fits and it doesn't, they'll take it back. Very good guys.

1 caveat to that is the gen 1 996 is slightly different in the rear... and Damon (tire rack) will tell you that on stock offsets the 295 is a close fit.

Great, thanks all for the input.

My clearance issue was better when I added more camber in the rear... at 2.5 deg the 295 would fit with room to spare. You are welcome Dell

so this is highly manufacturer and sidewall dependent, but the section width will decrease about 5mm for every 0.5" reduction in wheel width from the wheel width on which it's supposed to be mounted. a 295 is intended for a larger wheel than a 265, but not much, so you might see a reduction of the 295 of about 5 to 10 mm overall, but I believe it will still be 10mm wider in either direction, and might slide around a little bit at lower temps/pressures.

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i did not know that. 285 A6 hoosier on my OEM 10" wheel on my gen2 996 has lots of room to fit. i wonder what porsche did differently back there.

Adrian Streather sent me some notes on the slight differences in the rear end between mk1 / mk2 a couple of years ago, let me dig

Thanks I was looking for the notes I had on that as well.

295 is measured at the widest part of the tire mounted on the median approved wheel width properly inflated. Refer to the intended tire manufacturers website for correct diminsions on ideal wheel width. OR why not use 285/30/18 to enjoy increased gear multiplication over the taller 265/35/18, simplified installation, more even tread wear, reduced understeer, etc.

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found it, there are some bits that need to be reworked on 99-2001 cars for the 11 inch max offsets pm me your email address for the official porsche doc

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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.

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

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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.

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

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

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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.

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

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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.

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

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.

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