hamah

I'm confused about the correct tire size, and to some degree the correct wheel width, for 17" on my car.

Currently I have a set of Sport Classic II in 17" with 7" on the front running 205/50 and 9" on the rear running 255/40.

Wanting to go to a period correct wheel I recently bought a set of 17" Cup 1s with 7" on the front and 8" on the rear.

According to this document those size wheels are approved.

Interestingly the current tire size I'm running on the rear is correct for wheels of 8, 9, and 9.5" width according to this article.

Having said that I've seen other sources saying that 255/40-17 is for wheels of 8.5 to 10 inches. (an example is here)

1) Did I get correct sized Cup 1s to replace my Sport Classic IIs?
2) Is the 255/40 the correct size tire for 8" width wheels?
3) Assuming that the answer to the above 2 is "yes" will I notice a huge difference in handling (I am a "spirited" driver but not a racer and don't track the car) with the 8" rear vs. the 9"?
4) Probably not the place to start a discussion on which tire to get but I currently run Pirelli P Zero Rosso and don't like the ride/noise. I'm thinking about Michelin Pilot Sport PS2. Opinions? Suggestions?
5) I've also seen reference to running 225/45s on the front instead of 205/50s. This size does not seem to be "approved" and never shows when running fitment tools like those at Tire Rack. What is the feeling about deviating from the norm when it comes to tire sizes?

A lot of questions I know but I'm really scratching my head over this whole thing.

Jimjacqmx5

The stock size for 17's was a 205/50 front in the RS from memory but you wil find nearly everyone runs a 225/45 front and occasionally a 235/45 front although this has a larger rolling diameter and will lightly rub on full lock on the inside of the wheel well, dependant on wheel offset and camber settings.
Those running a 235 front usually run it with a 265/40/17 rear while those running a 225 front usually run a 255/40/17 rear which is probably the most common aftermarket size when upsizing to 17's.

I personally think the best tyre for raod, idf you aren't doing big kms and like the ultimate grip is the Dunlop Direzza Z1 which I think may be called a starspec in the US/Canada.
It has a racetyre carcass so has stiffer sidewalls giving much better turn in and feel. It also has a semislick rubber compund but runs a conventional tread patern to give good wet weather performance.
This tyre is similar to a Falken Azenis RT 615 or Bridgestone R-01 but betters them both.

Grip is far and away better than any 'road' tyre. Wear is still not too bad, count on 15-20,000kms per set.

If not, look to your local tyre tests in road and track etc. Michelin and Continental often are beaten in these comparisons by Goodyear and Bridgestone tyres which are usually cheaper. We have a Bridgestone in Oz called an Adrenaline which is excellent as is the goodyear eagle assymetric tyre. Both have been proven to offer more grip, better braking distance performance than the michelin and continentals.

mr pg

Not sure the statement above about 'nearly everybody runs 225/45's' is true, at least not in the UK. I suspect most still run the standard size.

jimq

I must be irregular. I run 205's

911Jetta

People normally post to see if 18" wheels fit...you are not really deviating at all.

1) Yes, you will have no problems with the wheels fitting correctly. I have a 7.5" rim in front which is also common with OEM styple replica rims.
2) Yes. That size came stock on some 964s.
3) (you are not tracking the car, so) You won't notice a difference. The tire will fit a little differently on a 8 or 9" (bulged or stretched)
4) Not going to touch that one...
5) 225s are a common upgrade on C4s to reduce understeer, same concept with C2s.

Enjoy your Cup1s, its a classic look.

hamah

So after searching a bunch of tire manufactures sites they all seem to say that 255/40-17 is NOT correct for an 8" rear wheel.

I've checked with 2 different people in the tire business whose opinion I trust and both have suggested I change the rear from 255/40-17 to 245/40-17 for the 8" wheel as this optimizes the flex point in the sidewall or some such thing (greek to me).

Both also suggest the change on the front from 205/50-17 to 225/45-17 if I think there is too much understeer currently.

Anyone running this size rear tire on a 8JX17 wheel?

race911

Uh......what did our RSA's come with stock on an 8" rear? http://www.rsamerica.net/OEMinfo/Tec...ndex.htm#tires

Jimjacqmx5

Sorry, I read through the first part and had a 9" rear rim in my mind as cup wheels usually come as 7 1/2" front and 9" rear. 52 offset front and 47 rear from memory.
A 255 on an 8" rim is a bit tight.
I have run 255 on 8.5" boxster/cayman rims and that was ok but 8" might be cutting it a bit fine.
I used to run a 245 rear on the original 16" rims I had for the first few months of ownership which were an 8" rim and that was fine..If that helps.

Nader Fotouhi

The info on the door says 205 F and 255 R for 7 and 9 17" wheels. FWIW, I have put 275 Hoosiers on 9" wheels on my race car. Some tires are have larger foot print (width) than others in the same size. The width is only one dimension. You have to consider the diameter of the F and R tires and keep them about the same. I use 225/45?/17 on F and 255/40/17 on R which are about 25" in diameter.

hamah

The guy I bought the Cup 1s from confirmed he had been using 255/40 as well. Very confusing. On the one hand this forum, various Porsche docs, and online sellers like Tire Rack and 1010tires.com say 255/40 but when you check the actual tire manufactures (including the ones that Tire Rack and 1010's fitment tools recommend) they say not for 8" width. Is it possible that N rated tires like the Pilot Sport PS2 are different in some way that allows the 255/40 to be mounted on 8JX17?...or is it more likely I'm making a mountain out of a mole hill? At any rate the tire calculator shows the various F&R sizes as follows. All look to be close....

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
205/50-17 4.0in 12.5in 25.1in 78.8in 804 0.0%
225/45-17 4.0in 12.5in 25.0in 78.5in 808 -0.4%
255/40-17 4.0in 12.5in 25.0in 78.6in 806 -0.2%
245/40-17 3.9in 12.4in 24.7in 77.6in 816 -1.4%

raspberryroadster

i think your tire guys are correct, 255 in the rear is more properly fitted to the 9 in. wheel....(i run that size on my stock 9J x 17 Cup 1)

race911

Mt. Everest out of flat land...............

I guess my post above was in "invisible;" but to reiterate, since the factory delivered the RS America with an 8" rear wheel and a 255 tire I fail to see where there's a problem.

Cupcar#12

255mm is the correct stock rear tire size.
not sure what the issue is.