Alloy Wheels (18") - Urgent!

Hi, I finally managed to track down an original set of 996 Carrera alloys!!! I'm going to pick them up at the weekend and I'm just triple checking everything....the only problem is that I think I've found a potential problem and need some advice. They are off a 996 GT3 mk1 and are original genuine Carrera 5-spoke alloys (6 years old). The dimensions are: rears - 18 10J offset ET65 with 265/35/18 tyres fronts - 18 8J offset ET50 with 225/40/18 tyres. According to the Porsche specification (to fit a standard Carrera/Targa), they should be (this is from my owners manual and the Tequipment book): rears - 18 10J offset ET65 with 285/30/18 tyres fronts - 18 8J offset ET50 with 225/40/18 tyres. As you know, I've been looking for absolutely ages, but I need to check that they are the right size. My questions are: a) Why do they appear to have different size tyres? Can you fit 285/30 ZR 18 to these wheels? b) What is the difference (265 versus 285 is width, but what does the 30 refer to...profile, ie. the wall height?) c) Do I have a problem fitting these to my car? I suspect the answer is: a) No b) ? c) Yes Would really appreciate a quick reply and the seller seems a really nice guy and I don't want to mess him about.

[Image-only or quote-only post; no author-written text.]

a) That was the tyre size fitted to the pre-facelift cars. b) Change the rears if you have any doubts. c) If the offsets are correct, you won't have any problems. Remember that the important dimension is where the wheel centre line lies in relation to the mating flange.

Hi, That's a relief....I think? Just to clarify this for me: a) Do you know why/how you can fit 285 wheels when they already have 265s on them? Wouldn't the alloy wheel need to be physically wider to accommodate the tyre? b) If I understand your %age description correctly, doesn't this mean that 265/35 are a physically "bigger" wheel, ie. has a larger diameter (by 5%)? Doesn't this affect things...ie potentially jacking the car up at the rear and slightly increasing the centre of gravity? c) The current owner subsequently fitted 15mm spacers which pushed the wheels right out (almost to the arches). I don't know if they had 5mm spacers on originally (I can check). BUT, I thought the GT3 mk1 was also narrow body, so why would it need spacers? Do you think this GT3 Carrera alloy wheel is physically the same as the standard 996 Carrera wheel please? Apart from the tyre sizes, the offsets seem identical (ET65 for my specification and what these alloys are listed as)...so this would suggest that I don't need spacers/longer bolts? Thanks for the quick reply, I really appreciate that.

One other thing, I've further read the manual and it lists two specification (summer) rear - 285/30 ZR 18 10J x 18 H2 (winter) rear - 265/35 R 18 10J x 18 H2 excluding the R rating, this does suggest that you can put 265/35s on the rear. I'm not going to track the car, but I do worry doing 70mph without the correct specification wheels. The rear wheels have 6mm tread on them, so would prefer to run these out before changing them if possible.

Each width wheel can safely carry a range of tyre widths. The main thing you are looking for is sidewalls that do not angle 'in' to meet the wheel rim. (easier to consider a cross section of the tyre/rim) See the table for the PS2 here for an example that also indicated rim width range and also up/down the table for different width tyres that would fit on the same rim width. http://www.tirerack.com/tires/Spec.jsp?make=Michelin&model=Pilot+Sport+PS2&tirePageLocQty= Whichever width tyres you use (285 or 265) you want to keep the rolling radius as consistent as possible, hence the varying profile heights for different width tyres. Have a look here to see how different tyres sizes change the rolling radius: http://www.miata.net/garage/tirecalc.html

OK, I see. Both the 285/30 and the 265/35 list as being in the 8.5 to 10.5" range. Gotcha, ok. The 35 is the profile, which is expressed a percentage of the width of the tyre. In other words, 35% of 265 (92.75) is similar in profile as 30% of 285 (85.5). So it seems as though the alloys off the GT3 mk1 are regular Carrera II 996 wheels. The easiest way to check this is to get the 996.xxx.xxx model number off the GT3 wheels and phone the OPC to get the Carrera II specification for a standard Carrera 996. Does anyone have a Porsche part number lookup table please? However, seeing as the offset and specification is the same, they are probably the same wheel?

No. There is no requirement to match the profile - the profile is simply just the profile of the tyre. It's the rolling radius that you want to keep constant. No - the Mk1 GT3 wheel was a different design to the 5 spoke Carrera wheels. But it just so happens that they are the same size/offset (+/- 2mm on the front) 5 spoke Carrera IIs: - 10J ET65 = 996.362.140.07 - 8J ET50 = 996.362.136.07 Mk1 GT3 SportDesigns: - 10J ET65 = 996.362.140.50 - 8J ET52 = 996.362.136.50

Great website (miata.net) It shows 265/35 (v 285/30): sidewall 4.4" (4.2") Radius 11.4" (11.2") Diameter 22.7" (22.4") Circumference 71.4" (70.5") Revs/mile 887 (899) Speedo = 60mph, Speed 60mph (59.2) - 1.3% difference So not much difference.

I think we're saying the same thing. The reason why the 35 changes to 30 is because the width is different and this is adjusted to keep the rolling radius similar. The gotcha comment was the penny had dropped about the profiile number changing in relation to the tyre width. Sorry, to be clear, these are Carrera 5 spoke on a GT3. What I was wondering was whether there is only one Carrera 5 spoke design, or whether there was 1 for the GT3 and 1 for the Carrera (or 1 for pre and 1 for post facelift). I think there's only one model personally....it's just simply that the tyre size changed on facelifted cars (as someone pointed out on here earlier) Thx for putting part number down,I can now check this against the ones I'm getting. Is there only 1 part number for 996 Carrera 5 spoke? Sorry about being a numpty here.

In my experience GT3's never had Carrera II alloys They all had Sport design alloys, the difference between Sport Designs on a Gt3 and sport designs on a Carrera 2/4 were the fronts were 8'' rims on a GT3 rather than 7.5'' on a carrera 2/4 The rear wheels on a facelift 3.6 Carrera 2/4 were either 285/30/18 on facelift cars 265/35/18 were the standard size for the rears of pre facelift carrera 2/4 with 18'' alloys ( this i have checked with an owners manual from a pre facelift) Carrera II's were only fitted to facelift cars from the factory

Ok, forgive me for being stupid but... Why is the rolling radius different for front and rear? Surely you would want the rolling radius the same? If I put my tyre stats in I get the info below. Why wouldn't tyre designers keep the radius front to back the same or is it just a case of best fit?

Hi Mark, that's a great response, thanks. So it sounds as though these were fitted as an aftermarket. Do you know if the pre- and post- facelift Carrera 5 spokes are the same alloy and part number....and the only difference is the tyre size? Ok. So this further supports the theory, that it's just the tyre size that changed.

^^ spot on. GD - the Carrera II alloys are a popular wheel to be used as a second set of track wheels: they are strong, Porsche OEM (so easy to resell), perfectly sized for the Mk1 GT3, and they are lightweight (lighter than the SportDesigns). Stuart, There's no real reason for the front and rear axles to have the same rolling radius. It's important to keep the rolling radius the same as standard (front or rear) because the ABS units detect rotational speed, you want to maintain relatively consistent handling the way Porsche intended, and you want to maintain consistency on the speedo readout.

There was only one 'Carrera 5 spoke' aka 'Carrera II' alloys for the 996 which was released with the facelift 996. Below are the Porsche OEM 18" wheels with part numbers, offsets, weights.