Different size rear tires on C4-S.....effect on AWD system....HELP

Was just about to purchase a set of track wheels and decided to call Tire Rack about some track tires. There is not an exact size match for Hoosiers (my choice for auto-x) for the rear which is 295/30/18. They only have 305's and 285's. In a previous post some people recommended I could go with either depending on how I wanted to affect the gear ration. I figured I would go with 285's for auto-x. Tire Rack recommended NOT going with anything different in terms of size except the stock sizes. Otherwise they said I risk damage to the AWD system. I figured it would be ok since it's the same axle, but not sure what to think now. I need to know soon since I am just about to purchase the wheels. TIA

The issue is with the diameter of the fronts vs the diameter of the rear tires. They need to be close. I don't remember how close but it should be in your manual or someone here should know. As I think I said before, you should be fine with 245/35/18 and 285/30/18 or 245/40/18 and 305/30/18. To be on the safe side, go to the "spec" section on Tire Rack and compare the diameters of the sizes these suggested.

Jim thanks for the reply. Yes I remember your previous comments but I want to be sure before I purchase the wheels. I just got off the phone with Hoosier and he gave me some options with the specs that he think might work. He was using a rule of thumb of not more than 3% difference from the stock sizes. By this I mean the Diameter, Treadwidth and circumference sizes. Not necessarily the sizes printed on the side of the tire. Is 3% a good rule to go by? Jim do you know where I can find those stock dimensions so I can compare to the Hoosier recommendations? * By the way the Hoosier guy recommended your first combination posted above....

On the experience and advice of people with twin turbo's who track their cars, I went with the 285's for the rear. The problem with AWD and tire sizes is the viscous coupling. You don't want to stray too far from the OEM specs. If you look at the diameter of the 295 and compare it with the diameter of the 285, the difference is negligible. If you look at the rolling distance per revolution, it is even more negligible. TT guys that track and use R-compounds that I've met use 285 for the rear. Either that or they fork out bucko bucks and go with the Corsa or the Pilot sport cups, which do have 295's. Want some more free advice? Unless you are very track experienced and want to spend a lot of time on the track, just go with the street tires. There's enough rubber in there to keep novice to advanced novice happy. Plus, you don't have to get a second set of wheels or beat up ypur current set of wheels by mounting and dismounting tires.

Lance, The Tire Rack site is a good source for tire specs. Just go to the page with your current tires and click on the "specs" button. It has all the info you will need. They also have the Hoosier specs so that you can compare. I don't really get the 3% from original comment. I think the real issue is the variance between front and back. The only guy I know that club races a 996 turbo uses the 245-305 combo so I know it works. That's also what I use on my C2 but of course I don't have awd to worry about. For auto-xing I would agree with the 245-285 combo. It will give you a lower gear ratio, more oversteer and allow you to throw the car around a little more. I have to disagree with Palting about track tires. Learning to read your tires is critical to understanding how to set up and drive your car and it's never been clear to me how anyone can do that when they use their street tires on the track. Just my 2 cents worth. Also, don't be tempted to buy MPSCs rather than Hoosiers unless you need to drive to the track. I used them for awhile and they just don't warm up fast enough to be used for auto-x. Again, just my opinion. Have fun. Jim

Bad experience, really BAD experience. Someone told my daughter she couldn't fit snowchains on the front her MY2000 AWD Chrysler T&C with the "+1" 235/60/16s. So she went out and bought two wheels and tires for the front only, 225/60/16s. Last Friday, about 60 miles down the road, up the pass actually, the rear PTO ring and pinion are stripped according to Chrysler. That's one hell of a tough VC. Belatedly she has found that when even replacing a worn tire some tire shops recommend shaving the replacement down to the level of the others.

Thanks for the feedback guys. Palting...I decided on the Hoosiers for the same reason Jim states. They are the best setup for auto-x. If I was doing more track I would probably go with MPSC's. Jim...I'm not sure if the Hoosier guy meant a 3% variation from front to back or a 3% variation from the stock sizes. In either case I want to be cautious. Here are the measurements he gave me that I want to compare. These would be based on an "option" that Hoosier gave me: Front 245/35/18 Rear 285/30/18 Diameter = 24.5 Diameter = 24.6 Treadwidth = 9.2 Treadwidth = 10.9 Circumference = 7 .1 Circumference = 77.3 Now if I compare the stock sizes and their corresponding diam., tread, and circum. and it's within the 3% I should be ok?

Update...I am thinking about going with 17" wheels to open up my tire choices. I found a set that have the following specs: Front: 9" wide, backspacing 180mm Rear: 11" wide, backspacing 190mm. I don't know what backspacing means and am no expert on offsets (same thing?) but would these sizes fit the 996 wide body? I called the seller and he said the 9" front will fit fine but would have to check the offsets to make sure they fit. These were originally on a 993 wide body. TIA

Holy schmolley. Talk about deviating from OEM specs. With 17" I have no idea. You're on your own, buddy. I just checked the specs to make sure I remembered my research correctly. Using MPSC as the model, diameter is 25.1" and 25.2" for the 285 and 295 respectively. One is 834 revolutions per mile, the other is 833. PZero 295 has a diam 25.0 and rpm 801. That's like the diff between a slightly worn tire vs a brand new tire. I doubt that the viscous coupling, PSM, ABD, XY and Z will be able to tell the difference. I bow to JimB's opinions with regards to track experience and advice. He's got the expertise, and I was raised to respect my elders (kidding!!) I also agree with the Hoosiers vs MPSC. Hoosiers warm up fast but don't last as long, MPSC's last longer but don't warm up fast.

And then you also must compute rolling circumference with the tires inflated properly. Or is it inflated properly so the rolling circumference will be equal front and rear? Are the OEM tires of equal rolling circumference or does Porsche use a fudge factor? Say slightly different final drive ratios in the front vs the rear.

Palting, Finally some respect and all I had to do to get it was grow old. Life isn't so hard after all. Lance, Backspacing refers to the measurement from the back of the wheel center (where the wheel contacts the hub) to the inside edge of the wheel. It is different than offset. If you took the width of your wheel, converted it to mm, divide by 2 and then added the offset, you would have backspacing in mm. I have no idea if the 17 inch wheels will fit. Someone else will probably know. Good luck. Jim

I think the biggest issue with 17" rims on the C4S is clearing the brakes. If you look at your front wheels, you'll see that there is not much clearance between the caliper and the wheels with the stock 18" wheels. -Scott

Thanks for all the great feedback. Does anyone know if the 17's will have a brake clearance issue on the C4-S?

you can NOT use 17" wheels on the C4S. you must use 18" wheels or they won't clear the brake calipers. actually i don't know why i'm using CAPS because as soon as you try and mount them you'll find out they don't evey fit over the calipers and still allow you to put in the wheel bolts!

for any AWD car, you should replace all of the tires if one or more goes and it has more than 20% wear. some are ok up to 25-30% wear difference, but i wouldn't try it. a tire is a lot cheaper than a new viscous coupling! any AWD car will have the tolerances listed in the owner's manual.

Thanks Teflon. I guess the 17's are out...

Teflon I just checked the ad of a guy who is selling 17"s and it states that the wheels will "clear any big red brake system". Are you sure the 17"s won't work? Thanks.

Maybe, the seller is referring to the older 993 red big brakes. Porsche does make it very clear that you cannot use 17" wheels on the C4S in their winter wheel and tire TSB. It shows that all 996 cars can use a 17" except the C4S, Turbo, GT2 and GT3.

Hmm... this is interesting. I have 17" snow tires for my 01 C4 that I was planning on keeping when I get my C4S. I'll plan on selling them now... good to know. Rob

I spoke to a Porsche tech briefly and he said that the 17" rim TSB may be valid but doesn't necessarily apply to ALL wheels i.e. aftermarket. So I guess if there is a 17" wheel that clears the calipers you could still use it.

Here are your options (based on lots of personal experience) I use: 245/35/18 plus 285/30/18 rear; OR 245/40/18 plus 305/30/18 rear. FOr slicks, I use 235/650 front and 280/650 rear or 305/645 rear. Bottom line, keep diameters as close to the same as possible.

Thanks RSR... If I use your first combination of 245/35/18 for the front and 285/30/18 in the Hoosier auto-x compound the overall diameters are 24.5" front and 24.6" rear. I believe that should be more than close enough. For comparison tire rack lists the diameters of my stock Pirelli Asymmetricos as 25.2 front and 25 rear. So if anything the Hoosiers are actually closer in size front to rear.

I didn't even see the auto-x reference..... for that I would definitely use the 285 rear setup for 2 reasons. 1) better setup vs the 305 rear to rotate the car; 2) shorter tire which provides better acceleration vs. taller 305 tire. Good luck.

My thinking exactly. What a great forum.

Obviously I agree with RSR (look back about 40 posts) with the exception of the notion that shorter tires accelerate faster. I only bring this up because it seems to be a common idea that doesn't hold up at the track. Certainly if you are accelerating from a given speed in the same gear the shorter tire will be faster but the difference between 285s and 305s is enough that you frequently run into situations where you can be a gear lower with 305s. The trick is to have the right final gear ratio, not the lowest. A 996 on 305s in second gear at 5800 rpms is a whole lot quicker than the same car on 285s in third at 4000 rpm. (these numbers are just made up to make a point. I have no idea what the actuals would be.) Having said that, 285s are still likely to be your best bet for auto-x. Now stop hanging out on this board and go buy your tires.