tire help: 225/45+295/35?

[vrex]

Hi all, im trying to locate a set a tires for my vehicle (2002 c4s, 6 speed). ive read a bunch of posts of various tire reviews, but havent run across any posts on what i am hoping to do. My right rear is showing chords (!!) and i have an open track day coming up on 5/29 so i need to get some tires in a hurry. i dont imagine i will pass tech with the current tires, nor would i want to chance it. let me put a few things out there:

i am a complete novice at racing cars. ive been karting for the last two years, but this is my first track event with the porsche. i wont be driving it insanely hard, i just want to learn how the car reacts at speed. that said, i wont need a crazy sticky tire. my porsche is my only car, i daily drive it, so i need something with decent tread wear rating. i dont care to do ps2's. on my list are conti's, kumho spt, and the the bfgoodrich kdw2. my question is concerning the kdw2. it comes in a 295/35 instead of 295/30. they also make a 225/45, so i could potentially have a matched set there. if i upped the tire size all around, will this adversely effect the AWD system? damage to the drive train is my biggest concern right now. there are alot of tires in 295/35 that are interesting to me, the kumho MX included. any pointers would be appreciated.

Best-

[jgriffin56]

By increasing your tires to 225/45 Front and 295/35 Rear is only going to make the tire taller, thus losing the "low profile" look. Thr first number is the tire width the touches the road, and the second number is the tire height. I really don't see much good in going with taller tires for tracking making the car sit higher, and adding the weight as well. And to tell you the truth I think it would make the care look as a little tire heavy. If you go the other way and add tire width, then you risk hitting the tire wells and any internal suspension parts. I think the stock tire on a C4S is big enough ) I too have an 04 C4S, and if anything I would maybe go a little wider on the front to a 235 and maybe a 305 on the rear it it fits, I would also check the Tire Rack and speak to them. ALso the track may not let you run non-N2 tires on a Porsche, I don't know, but better to check first.

[vrex]

not to worried about aesthetics, more so with gearing. taller tire= taller gearing, worse acceleration and whatnot. more important to me is not damaging the awd system. AFAIK, tracks dont care what kind of tire you run, just that the tires are in proper condition. ill have to look into 235/305's, they'll probably fit the stock wheels, if not a bit rounded out. i havent chatted with anyone over at tirerack yet, planning on doing it today. thanks for the help!

[BruceP]

You can calculate the rolling circumference of the tire approximately like this:

The second number - 35 or 45 in your case - is the height of the sidewall expressed as a percentage of the width of the tire, in millimeters. Calculate that. For a 225/45, for example, that number would be 101.25 mm. Then add your wheel size, converted to the same unit of measure. E.G. an 18" wheel is 457mm. Divide by two for the radius. The total radius in this example is about 330mm. Use the 2 x pi x r formula to get your theoretical circumference.

Compare the two tire circumferences in terms of the percentage difference between stock and the size you're considering, and you'll get a good sense of whether you're affecting gearing, speedo accuracy, rotational speed differentials (which matter to stability control systems) and, in extreme cases, even suspension geometry. I wouldn't lose any sleep over a 5% difference, say.

It seems fussy, but it's a more consistent way to compare tires sizes.

Edited May 25, 2009 by BruceP

[smackboy1]

You can compare different tire sizes using this calculator

http://www.1010tires.com/TireSizeCalculator.asp

295/35R18 is only 1.04% bigger diameter than the stock 285/35R18. As long as the wheels can handle the tire then it should be OK. The general rule is +/- up to 3% diameter change is OK.

Another thing to keep in mind is that the stock 225 front, 285 rear understeers quite a bit. Going to 295 in the rears if it does anything, will make it understeer more. Might consider running 235 front, 295 rear.

Small changes in tire size like this are not going to mess up your AWD. Porsche uses a viscous slip center diff so there is no solid physical connection between the front and rear axles. Plus the system is set up to provide a minimum of 5% drive to the front axle all the time so there is almost always a speed difference in the center diff. A 1-3% difference in tire diameter isn't going to break anything. More important is to make sure the wheel well and the wheel can handle the tire. Oh, and the speedo will be a bit off.

[auto-xr]

Hi all, im trying to locate a set a tires for my vehicle (2002 c4s, 6 speed). ive read a bunch of posts of various tire reviews, but havent run across any posts on what i am hoping to do. My right rear is showing chords (!!) and i have an open track day coming up on 5/29 so i need to get some tires in a hurry. i dont imagine i will pass tech with the current tires, nor would i want to chance it. let me put a few things out there:

i am a complete novice at racing cars. ive been karting for the last two years, but this is my first track event with the porsche. i wont be driving it insanely hard, i just want to learn how the car reacts at speed. that said, i wont need a crazy sticky tire. my porsche is my only car, i daily drive it, so i need something with decent tread wear rating. i dont care to do ps2's. on my list are conti's, kumho spt, and the the bfgoodrich kdw2. my question is concerning the kdw2. it comes in a 295/35 instead of 295/30. they also make a 225/45, so i could potentially have a matched set there. if i upped the tire size all around, will this adversely effect the AWD system? damage to the drive train is my biggest concern right now. there are alot of tires in 295/35 that are interesting to me, the kumho MX included. any pointers would be appreciated.

Best-

996 tire spec is 25"overall tire diameter. 255/35/18 and 295/30/18. Going taller may rub, will increase center of gravity, and could hurt acceleration.

Edited May 31, 2009 by auto-xr