I just emailed TireRack with a request for price and availability of tires:

Question: I have a modded Turbo (500 HP)with a Bilstien Suspension set to
ROW height. I have Fikse FM-10 wheels 18X8.5 front and 18X11 rear. I want
to try Pirelli P-Zero Asymetrico's in a 295.30.18 for the rear. What size
should I use for the front? 235.35.18 or 245.40.18? I was thinking
245.35.35.18, but nobody seems to make these. Car is driven on street and
track and I want a "maximum performance" tire. Any suggestions appreciated.

Here was the response:

Hi Scott and thanks for the email,
The largest sizes I can recommend for the car would be 225/40/18 in the
front and 285/30/18 in the rear. According to the Porsche specifications the
tires you are asking for would not fit correctly. The best tire
recommendation I can make would be the Bridgestone Potenza S03 tires.

Oh well, I'll keep trying....

 

Scott,

I wonder if "Wheel Enhancement" (Culver City) might have some better information on tire sizes and brands. However, if you get stuck, I know someone who would swap you straight across for those crappy Fiske wheels and bald tires.
How's the CEL coming along?

 

CEL is on, but the car hasn't repeated the judder power loss thing. I guess I'll know in two weeks.

 

Next thing they are going to say is to run 36/44 for your tire pressures!

 

Scott,

I also had no luck with Tire Rack's tech support in the past. I think they are too risk-averse to suggest anything other than stock sizes.

I currently run 245/35-18 Pirelli P-Zero C's up front (with 285/30-18 in the rear), and they are great. I do agree that it's unfortunately an oddball size that is difficult to source. BTW, I've got them mounted on FM-10's 8.5" front and 10.5" rear; great minds think alike, aye? <img border="0" title="" alt="[Wink]" src="wink.gif" />

Using the <a href="http://www.powerdog.com/tiresize.cgi" target="_blank">Power Dog Calculator</a> , 245/40-18 and 295/30-18 tires should have a 25.7 in and 25.0 in diameter, respectively, and a 80.8 in and 78.4 in rolling circumference, respectively (a 2.9% difference). However, there are variances between different tire manufacturers, so you should try to look up the actual diameters and circumferences at the respective tire manufacturer websites to see if they are a good match.

I've noticed that the hot 996 models (e.g. GT3) seem to mate a 235 to a 295. I'm not sure why they avoid a 245 up front, unless it's because it's a difficult tire size to source.

Hope this helps.

 

Scott - give the Tirerack guys a break - I am sure they have learned their lesson about suggesting agressive tire sizes many times over. They need to be conservative to cover their *****.

The wheels you are running are the same size as the stock wheels on the 996TT - 18x8.5 front; 18x11 rear. I have a set of these and the OE tires are 225/40 and 295/30. So you know your rear tire choice is fine. As for the fronts, I like the 225/40s - but if you want a little more rubber, perhaps Anir's suggestion is worth a try.

p.s. I am the guy with 993TT set up very much like yours - I am putting down about 520 HP.

 

Scott,

I'm not sure if this info will apply, since it relates to 17" wheels, but here goes:

Domestic manufacturers often use a 245x45x17 on an 8" wheel width, and a 275x40x17 on a 9" wheel width. I use a 315x35x17 on an 11" width. BTW, I have seen 2 different brands of 315s that vary by over an inch in actual tread width.

It seems to me, based on this experience, that the tire sizes you are suggesting are proper, if even a touch conservative for those widths. Any trouble squeezing them under the body work?

 

Bill and Anir,
Great, I was going to PM both of you:
MarkD originally bought the wheels intending to run 305 and 245 Hoosiers on it, if memory serves.
The stock 285 and 225 tires look a little stretched out on them.
I'm using the car as a daily driver and intend to track it as often as time allows.
My intention was to get wider on the back and then increase the front to keep the balance if possible. I'm not ready to go Michelin Pilot Sport Cups because I don't have enough experience yet. (I'm thinking I need 5 or 6 good DE's under my belt before I move up to that.)
That being said, I figure Pirelli P-Zero Asymetrico's would fit the bill.
Assuming I go with a 295 on the rear diameter =25"
235.35.18 diameter = 24.6
245.40.18 diameter = 25.6

From your input, maybe the 235.35.18 is the way to go.

By the way my SO-2s now have 13,000 miles on them and 4/32 of tread left. I'm intending to use them at Willow Springs big track next month for my second DE. (I used the Kuhmo R tires for my first DE and I think they were too much tire for my skill level.) My concern is the SO-2s seem to have gone off a bit. The rear feels kind of loose these days, and steps out at the drop of a hat. Anir, I noticed that you had reported a "greasy" feel at 8000 miles. I'm wondering if I should use them as a learning experience at Willow or if I'm asking for trouble?

Thanks for the input!

 

Scott,

I think you'd enjoy Willow a lot more, and be safer and faster, if you chuck the older tires. If they feel greasy on the street, they may feel even worse on the track.

Let us know what you end up mounting. Have a great weekend.

Anir

 

Scott - as an aside, I have seen new 996TT take offs (wheels and tires) for as low as $1800. I run these on my 930 (using the OEM Conti Sport 2s) and really like them. Lots of meat in the back and a good balanced feel up front. Of course this is without power assist steering and pure 2 wheel drive - but if they feel good on the 930, I would think they would feel at least as good on the TT. I have thought about getting a set for the 993TT as well as I will need new tires soon and figure at least $1200 just for tires, mounting and balancing.

My wife and I do almost all of the RoadOne Racing DEs and will be at Willow Springs in March. Your buddy, Mark D, was an instructor at the last one.

 

Bill, I look forward to meeting you and your wife in March.
I'm going to run up to the tire store at lunch time and see what they can do for me. I think I'm going to take Anir's advice and get new tires. I still can't decide between 235 and 245 up front.

 

OK. I just got the Pirellis installed. I went with 235.35.18 and 295.30.18. I've only driven them 10 miles, but they seem a little numb. They don't seem to turn in as sharply as the SO-2s. The noise over rain grooved pavement is astonishingly loud.
I'm getting an alignment next week. (I might get the suspension set a little more agressively if it still feels the same.) I went with stock ROW settings when I bought the car, would rs specs be to extreme for a daily driver?

 

Scott - I am not sure if this applies, but you may want to try to do your own version of heat shaving. I always feel my tires grip better after hard runs. So maybe you can find a place to run the car hard for a bit and get those babies heated up - that may improve the grip and supposedly also makes the tread wear better and longer.

 

Don't forget that new tires need to be broken in...well, you dont have to drive differently, but it takes a little while before they start to grip 100%.

 

OK I ran them hard up Mullholland to heat em up. No change.
I went in to get it aligned. The alignment was way out! I had it set to RS specs and its very crisp now.
Grip still seems lower that the SO-2s, but they ARE full tread at this point. I can invoke the ABS WAY to easily