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.

 

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.

 

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