I parked next to a beautiful midnight blue “911” badged 996 yesterday. It was post 01 face lift with a Turbo/C4S front bumper, 996 rear bumper, quad tips, and stock-looking “Sport Edition” rims. The rears were shod with 295 40 ZR18 Michelins. I have wanted to add that little extra track in the back by moving up from the 285s to 295s myself, but have been told that tire will not work on the stock 10 inch rim. So, was the car I saw an Anniversary Edition which came stock with 11 inch wide Sport Edition rims, an aftermarket 11 inch rim which looks exactly like Porsche “Sport Edition” edition, or has the owner done the impossible/unauthorized and mounted a 295 on a 10 inch rim? if the latter, what are the implications, other than looking really cool.

If anyone can shed some light on this before I replace my rear tires, I’d very much appreciate it….

 

I think the 40 series would have too much sidewall and look like donuts. The correct size tires on a widebody 996 (C4S or TT) is 295/30/18 on 18"x11" wheels. I think you need 10.5" wide wheels minimum to safely mount 295's.
FYI: my tire sizes on my C4S are (BTW Porsche approved sizing and N-rated tires only):
summer: F 225/40/18 (8" x18 wheel) R 295/30/18 (11"x18 wheel)
winter: F 235/40/18 (8" x18 wheel) R 295/35/18 (11"x18 wheel)

 

The car probably had 997 18" Carrera III's which look alot like the 996 Anniversary Carrera Lightweights. The 997 also runs 295 40 18's, so its safe to say they were 997 takeoffs....They come in a 10 or 11" rear....the 11" rear would take a 295 tire.

 

I was told by porsche that 295 should not be mounted an 10" rim. 285 is max.

 

I have driven around Europe for three years with over 40,000 kms and many laps on the NUrburgring with 295/35ZR18 rear tires. I have had one set of conti contact 2's and a set of P-zero's. Both were on my 10 inch rear turbo wheel from a narrow bodied 996. The car has been serviced many times at Ruf Germany with no comments by them about the tire size.

 

I knew it!! I wasn't dreaming... I saw the same car again today and looked at the tires again - 295/40ZR18. On a 10 inch rim. How much of a difference does that 5 mm in the aspect ratio make between yours and the tires I saw? They looked nice and fat without too much sidewall height. And, what I liked most was that the tire stuck out past the rim just slightly. I don't track my car. Maybe someday I will, but for now, the worst I do is little blasts to 140 or 150 on a lone stretch of interstate, or up and out of the top of 4th gear from the mouth of a toll booth. The rest of my driving, while I don’t commute, is pretty mundane. I know, what a waste, right?

I know the tire guys are probably going to stick to the 285 rule citing sidewall issues related to safety, or a Porsche commandment. Or, is it something else? If you’ve done many laps on the NUrburgring under the watchful eye of Ruf, on a set of 295/35/ZR18s, and are still here to talk about it then I think I’ll be okay in south Florida. I think in my case looks outweigh performance.

 

I have a set w 295/30's and they are wide, but fine on the 10". I normally run 285's on the rear. I don't have PSM so it's not an issue of different diameters. I'd suspect 40 series would screw things up unless you changed your fronts too.

 

This is probably open to debate, but using the tire size calculator on 1010tires.com ...

Starting with a stock rear tire size of 285/30/18 on a 2002+ 996, I found the following:

295/30/18 (Speedometer Difference: 0.924% too slow, Diameter Difference: 0.93%)
295/35/18 (Speedometer Difference: 5.613% too slow, Diameter Difference: 5.33%)
295/40/18 (Speedometer Difference: 10.35% too slow, Diameter Difference: 9.39%)

You get similar numbers even if you start with a 265/35/18 tire...

The site does not recommend a diameter difference over 3%. For me, there is no debate. 295/30/18 should be the right tire size.

Also, the middle number, the aspect ratio, is not measured in mm. It is sidewall height divided by width of tire (keeping the unit of measurement the same of course). If you keep the same width tire and go with a higher aspect ratio, you're just asking for more sidewall. []

 

It's not recommended to mount a 295 on a 10" wheel for a reason. You could safely do it for years, but it only takes one problem to kill you.

If you're spending this much money on a car, go with the right tire/rim size combo. It's not like a 295 is going to give you that much grip over a 285 anyway. It's only a 3.4% difference in width. It doesn't necessarily increase your rear grip by 3.4% either since the PSI on the road is now proportially less with the wider tire. Even if it did, 99.99% of drivers will never be able to drive their car well enough to use an extra 3.4% grip.

 

I would like to know what that reason is, and does fluting it necessarily mean death? My car has 285s; parked next to the car with 295s there was a very noticeable difference in tire width. It gave the car a more aggressive look that was very appealing to me. You are spot on with the perceptible difference in grip, so really, the point is moot. Naturally, I’m concerned about safety. I’m hoping to find someone who can give me a factual explanation as to if and why such an increase in tire width could prove lethal. If the difference is merely scavenging seconds on a racetrack, then I’d definitely go for the wider tires when the current tires need to be replaced.

 

I suspect the ride on that 295/30/18 would be an the harsh side as compared to the 295/40/18.

I would love to be able to see an example of each of those three sizes mounted up for comparison....

 

How do you know for sure its a 10" rim? What wheel is it? Porsche makes many wheels in 10" OR 11" widths....so there is a possibility that the 295 is on a 11" wheel....I have 295's on 11" rims and I dont see how it would work well on a 10" rim...I had 285's on my 10's and that was the widest I would go....FWIW it HANDLED better on 265's and a 10" rim....ALOT less understeer...The 295's on 11's is great, but I also have wider fronts as well....with a 40 series tire I dont think it would handle all that well...too much sidewall on the rear.

 

Mine are 10's and I'm not concerned. There is minimal (I mean very minimal) "Bulge" withe the 295's. It's only 10 mm for heavens sake and it's only on a street set I rarely use. I can feel a diffence, but that most likely it's due to Hoosier and Pilot Sport Cups in 285/30's, versus a Conti 295. Tire width makes a huge difference. I ran Yokohama Slicks in a 265 when I was short on tires and the 285 equivalent (they measure them with weird numbers) had significantly more grip. 285's are big enough for the 996's, especially with a 5mm spacer, just make sure you use longer bolts.

Good luck

 

You can mount with whatever aspect ratio you want. I was just illustrating that a 295/40/18 is not recommended because it jacks with your speedometer. The stock wheels come with 285/30/18, so I do not know why you would want to add 10% aspect ratio and go 10mm wider at the same time. To each his own. Good luck with the set-up.

 

here you can read about the 4% rule. http://www.pca.org/tech/tech_qa_ques...3-66F33F9F8468}

You shouldn't deviate the front and rears more than 4% of each other. They will confuse you ABS, PSM and speedo setup. These cars are design as a 25" diameter tire, the farther you stray from it, the less dependable your braking, handling, comfort etc will be.

Here is a tire size calculator and see for yourself. http://www.miata.net/garage/tirecalc.html