bill930p

I will be fitting 18" Gemballa's on the Ruf soon. The rears are 18x10. They came off my 993tt. I'm assuming I'll have to use spacers.

My question is; what's the widest tire I can mount on the back. I was using 285/30/18 on the 993. Can I continue with the same tire, or should I go to a smaller size, such as those used by the 3.6 turbos?

nathanUK '81 930 G50

I have 285's on 10" wheels on my 930. They look a great fit with no rubbing.
There is even room for a wider tyre if you wanted to get a wider rim.

JBL930

I thought you could go up to a 315 on a 10"? I only read that on the net though so don't flame me if it's wrong

ZX3tuning

most 315s have a range of 10.5-11.5 calling for an 11" as ideal fitment.
you could probably run a 315 on a 10 but there's be a little pinch or bulge to how it sat on the wheel.
I run a 315 on a 11.5/18 for example and its just slightly stretched beyond a squared up tire. I also do not get any rub in the rear even under heavy squat on acceleration

assuming you space the wheel properly you should have no problems making a 315 fit without worry of rubbing but you'd be on .5" too small of a wheel... i personally don't think this is a problem as i've have put tires on wheels both 1" smaller and 1" bigger than what the manufacturer recommends, just understand there is no warranty and it does increase the chances(although very slim) of popping a bead free... more so on smaller tire bigger wheel(stretch)

nathanUK '81 930 G50

285's look perfect on a 10" wheel. I guess you could fit on a 295 but why bother to overtyre a wheel, it won't help cornering.

JBL930

I always wonder why people fit tires so narrow that the lip of the wheel is the first thing to grind against the curb, as long as the tire sits over the edge of the wheel far enough to protect the wheel then i'd be happy.
I've been looking at Ruf wheels for my 930, as you no doubt know these are 10" in the rear and i was just looking at the biggest tire i could fit, a calculator i found came up with 315 being the absolute widest.
The other reason of course is for it to look as close to a steam roller as possible

Metal Guru

It's the ghetto look.
The PO of my car had stock tires fitted and liked that look. I went with 235's in the front and 285's in the rear. Like it much better

JFairman

"I always wonder why people fit tires so narrow that the lip of the wheel is the first thing to grind against the curb"

Some people do that for the same reason many imsa race cars tires are the same way... you get less sidewall flex on turn in giving quicker steering response and better feedback of what the tire is doing through the steering wheel when cornering.
and, some people like the way it looks and/or if the rims are wider or more negative offset than the car was designed for it will lessen the chance of the tire tread and sidewall rubbing on the fender lip.

Putting a wider tire than the rim is designed for won't always give a bigger tire contact patch on the road surface in many cases because the edges or shoulder of the tire tread is being curled upward away from the road as the sidewall and bead is being pulled inward because the tire is too wide for the rim.
In extreme cases that can cause radial ply seperation causeing a lump in the tread that will make a rough ride and then blow out.
Not saying a 315 0n a rim designed for a 285 is going to always do that but it could.... and the tire will run hotter than normal from the stress.

Guns951

The following are 315's on a 17" x 11.5" rim (front wheel), the sidewall ratio is just about perfect, so putting these same tires on a wheel 1.5 incehs smaller would not look right.

The turbo body can take 11" rears no problem, I had that setup before I started this project, however for a 315 you need at least an 11" rim.

I'm ending up downsizing the front wheels 1 inch to 10.5 and I'll probably go with a 295mm width to maintain the correct aspect and fitment.

JFairman is correct, too wide a tire on a rim will not give better/bigger contact patch, you'll just end up with ballooned looks and a tire that isn't setup to give its best.

That's why I'm doing surgery on my rear wheels to make them fit the tires properly. I'm operating on the edge of the performance envelope, as such things have to be just right - so that's how I approach the problems my project presents.

Hope this helps someone.

nathanUK '81 930 G50

Those 315's look right on that 11.5" wheel

285 on a 10.5 should be nice and tight

I have 225's on a 8.5" wheel up front, they are really tight!
I think 235's would probably look better. Does anyone know if the car would corner better too? I'm sure straight line braking would be improved.

ZX3tuning

Depends on the cars you are seeing it on.
on VWs and most sport compacts it's a style/looks thing more than anything else. it allows you to put the widest deepest sdished wheels possible on the ca while minimizing the rubbing withou little to no fender work required.

on higher end cars like BMWs Mercs and even some Porsche's again it's a style thing but it does have a slight performance advantage although pretty minor on a car that weighs over 4000lbs like on some of the bigger BMWs and Mercs...

on drift cars it's super common as a stretched tire pretty much eliminates sidewall flex allowing for fast and smooth breakaway with little to no tire correction/defelction when the tire begins to gain traction. a squared up tire will usually have a break away point where it will let go suddenly(due to side wall flex over coming the lateral traction) and a point where it can snap back suddenly(where side wall flex comes back into line and traction increases). this gives a very twitchy feel to drift cars and oddly enough makes them harder to control(while technically "out of control"). stretching the tire eliminates this as side wall flex is basically already installed into the wheel/tire combo... think of it like a preloaded bearing.

there are lots of Myths/rumors about stretching tires being unsafe or dangerous. while all myths/rumors are based on some truth they often get blown out of proportion by people playing the broken telephone game....

reducing the sidewall height, like putting ultra low profile tires on, will increase the chaces of bending a rim... but unless you are an idiot and driving INTO potholes or aim for pavement heaves then this shouldn't be an issue.

strteching the tire increases the chances of poping the bead free... this is true but only under extreme cornering loads or from hitting potholes and pavement heaves. again, common sense....

makes it easier to curb your rims.... sure if you are not paying attention while parking, i can see that or if you let your wife/gf/mistress/kids drive your baby with your $1500/wheel Fikse/Kinesis/etc wheels... you almost deserve to have them curbed!


285s on a 10 is perfect. get wider wheels to get wider tires!

nathanUK '81 930 G50

I had some chaps that build race cars work on my 3.2 once. I asked them what they thought about the width of the tyres on my rims and they told me that in their opinion Porsche had already maxed out the size of the tyre compared to the rim and that it was fine. They also went on to tell me that in certain race series the rules stipulated what tyre had to be used with certain wheel sizes, they explained that they did this to keep the cornering speed of the cars down by overtyring the rims.

RarlyL8

It is important to note the variance of actual tread widths for a stated size per manufacturer.
Pirelli is notorious for short changing the tire size resulting in a significantly more narrow tread width when compared to other manufacturers tires of the same stated size.
You can properly fit a 315mm Pirelli on a 10" wide rim. All other manufacturers require a 10.5-12.0" rim. When measured on the same rim I found a full 1/2" difference between Pirelli and Yokohama. A larger size yielded a full 3/4" difference.
Do take this into consideration before purchasing any tire.

ZX3tuning

^^^^^^^^ that's true.
actaully i have seen big differences over every brand
some bigger than spec, some smaller and some correct.
it can even be different from model to model from the same manufacturer!

honestly, just buy whatever and run it.
there is more than enough room on a 930 to run anything smaller than a 315 without getting into fitment issues from any brand.

GL!!

Jitters

I run 295/30's Michelin PS2's on the rear and have had no problems at all.

Jitters