Fellas,
I have a few unanswered questions about tires. Does anyone know why Porsche has chosen to vary the rear tire size?? Why 295 and 305's? They are both fit on the same size rim. Do you need spacers to go to the bigger size? Do the rims that Porsches come with vary between a wide body and a narrow body car? Also, has anyone gone to 245's up front with the 305's in the rear?? Also, does it surprise anyone that Porsche hasn't officially N rated the Michelin super sports?? I am assuming they will be coming standard on the 991's. With the better performance (as Michelin has showed) it only helps porsche show how much "better" the 991 is as compared to the older 997.2. Thoughts??
J

 

Size/model/country availability maybe? Not all same model cars in the same year came with the same tire brands/models so it is probably a question of availability and/or pricing.

For example, AFAIK, some 2009 cars got shipped with Pirelli's, some with Michelins, and some with Continentals.

Yes, they both fit. In my car user's manual 2009 C2S, Porsche says the following sizes are approved for summer tires:

235/40/18
265/40/18

235/35/19
295/30/19

235/35/19
305/30/19


No. To my knowledge, unless you really play with camber and caster and end up with fronts rubbing the liners when turning, you should not need spacers just for wider tires...

Yes. Different width and offsets.

You need to run the numbers using a calculator to get the net diameters , I believe the rule of thumb is to keep the front/rear within a certain diameter percentage (4%) of what Porsche specc'ed out to keep ABS running properly.

Why do you want to deviate from the recommended combinations? Changing sizes will also change the car behavior, for example, if instead of running 235/305 you are running 245/305 you will get a front that is a little taller and a little wider so you will have a little more grip up front and hence, less understeer... is that something you want?

Tires is a never ending topic, someone else who has forgotten more than I will ever know can probably chime in with more info...

PSS are relatively new, certification takes time.

If it was only a question of performance, why not PSC or slicks? I am sure there are other variables like versatility, safety, average wear, price, etc.

HTH,

T.

 

I went with Super Sports in 245.35.19 and 305.30.19 on a set of Forgelines. Less push was my reason for going with the 245s in front. I like the way my car handles.

 

The narrow body and wide body cars use the same rim widths, just different rear offsets. Some of the factory wheels have 11.5 or 12 inch wide rear rims, so use 305/19 tires, vs the 295/19 tires used on the more standard 11" wide rear rims.

 

dont know why the put 295 and 305 on 11s

 

I asked about the "different rim" question because when ordering a new P-car, there are many rims that can be chosen. Does Porsche really have a narrow and a wide version of each rim? Seems illogical. And often I hear the wide body guys wanting to go with big rear spacers to get the tires out more. So I am still confused about rim offset. Also, in the wide body cars, 305's were typically on the rear. I would've assumed, that with a wider rear track, and wider tires, the car would tend to understeer more, maybe I am wrong. When my 09 C2S was built, it came optioned with "turbo" wheels and 5 mm spacers, 235's up front, and 295 rear. Without measuring, I am not sure what I really have (width, offest, etc). I would like to go 245's and 305s when the Michelin SS get the N-rating from Porsche, and from what it sounds like, I could. I understand that changing the tires could alter the handling, but I would be adding 10 mm width at all four corners. Besides guessing, does anyone really know?

 

My C4S came with 11" rims and 305/30/19 tires from the factory.

 

The AWD cars usually use a 305, and some wheel choices are 11.5" that necessitates the 305. The RWD cars usually got the 295. It's not concrete, but the usual.

 

Thanks for the clarification Damon. It is interesting that the AWD cars came with 305 rear tires that are 2.86% larger diameter than the fronts, when 295 rears are closer in size to the front tire diameters, being only 1.9% larger. I would have thought the closest match would be preferable for AWD.

 

I think I remember reading that the guideline is 4% or less.

 

Exactly what I want to do, for the same reasons. Any rub up front?? What is the rim size and offset of your Forgelines?/