Tire radius front vs. back for Carrera 4S (new rear tires)

Greetings For Christmas, I ran over a nail in my right rear tire. Then, I learned the hard way that my 996 doesn't monitor tire pressure, which was a bit of a surprise. I noticed the tire was low after driving on the highway with it nearly flat. The tire place confirmed that the sidewall was damaged and I got to buy new Michelin Pilot Super Sport tires for the rears. However, on my first run out it feels like the car is straining a bit - it definitely could be my imagination, but remembering that the front/rear radius needs to be within a certain % for the viscous clutch in the AWD system I wanted to get some input prior to driving anymore. Here's the info. 2003 996 4S with 997 wheels (19") front and back. All are Michelin Pilot Super Sport Rear tires: new 295/30/R19 Front tires: not new 235/35/R19 I took some tread depth measurements on the new rear tires and they were roughly 7.1mm where I was measuring. I kept the non-damaged old rear and measured it was well in a similar spot and it was roughly 4.7mm. The front tire measured about 4.6mm in depth. I am really hoping this isn't an issue, or that maybe adjusting tire pressure could keep me in spec. However, I'd rather know now if I need to bite the bullet and get new front tires as well vs. destroying my viscious clutch and AWD system. Any help would be appreciated. Thanks and happy holidays! Charlie

Or the issue might be driving on High Performance Summer tires in the Winter.......just sayin'

Since you're in Ohio this could be the issue^. Pilot sports (what I'm running are not really meant to be driven below 55 degrees. The other thing that can be feeling the strain is the 19s. More rolling mass will effect the car, but you should be used to the "straining" feeling by now since you're driving the car already. If I were you I would get a new rear for the left to keep it consistent of 4.6 in front and fresh rears out back. The rear tires on these cars go probably twice as fast as the fronts due to the additional weight being on top of them. Many people here replace the rears of the car and keep the fronts.

There are plenty of people running 19” rims on their 996 C4S so I will be interested in what size tires they use. I am not an expert (Internet, who really is?) but I have heard a maximum of 3% is OK for the viscous coupling. Your 235/35-19 and 295/30-19 tires are 2.0% too large in the back. With partially worn fronts you are at 2.7%. If the fronts were completely bald, you would be at 4.4%. Fortunately, this never happens and the rears will get smaller sooner than the fronts so the 2.7% will be reduced as time goes on. People should feel free to check my numbers. My winter setup is 235/40-18 and 285/35-18 which puts me at 2.0%. If I were to replace the rears I probably would drop back to 275/35-18 but, so far, I have 3600 miles on them with no apparent problem. It looks to me that you are probably ok but you could get 225/40-19s for the front and not have to think about it again.

According to the calculator, your diameter variance is 1.93%. I do not know the variance recommendation for your specific model, it's "typically" 3% for both the effect on the braking system and the viscous coupling on the AWD. http://www.1010tires.com/Tools/Tire-...5R19/295-30R19

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Curious as to where you are getting the 55 degrees from. All the language I seen states "While Pilot Super Sport tires are designed to allow sports cars, sporty coupes, performance sedans and supercars to achieve their full potential in dry and wet conditions, like all summer tires they are not intended to be driven in near-freezing temperatures, through snow or on ice." I have driven my Porsche that had MPSS in the mid 30's and up with no issues, dry or wet. Snow or ice, forget about it.

Although there is little technical information available, most of us agree that the AWD system will not work properly if the rear tires are larger than the front. The nature of the viscus differential requires the rear tires to spin faster than the front in order to transfer power from rear to front. There are a few dozen posts about this.

^^^^^^^^ Good point. On my 1996 C4S the stock OEM size the diameter is less in the rear, -1.43%. http://www.1010tires.com/Tools/Tire-...0R18/285-30R18

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You should not use air pressure to try to compensate for incorrect diameter tires. Let's see what the tire size number mean in revs/mile: 235/35x19 = 792 295/30x19 = 777 The rear tires make fewer revs/mile than the fronts. Thus there is no torque transfer through the viscous coupling from the rear axle to the front axle. In fact just the opposite. The news doesn't get better. Here is a factory sanctioned tire/wheel set's numbers: 225/40x18 = 804 295/30x18 = 808 With the wheel/tire combination you have on your car now the % difference between the revs/mile of the front wheels/tires vs. the rear wheels/tires is -1.8%. With the factory wheels/tires the % difference between the revs/mile of the front wheels/tires vs. the rear wheels/tires is +0.49%. Thus not only are the front wheels/tires turning faster, making more revs/mile than the fronts they are doing so at over 3x the ratio of the factory wheel/tires combination. Based on my working the numbers the 19" wheel/tire combination you have has not only the torque transfer going the "wrong" way but is doing this with a rear/rear tire speed difference that is quite a ways outside the speed difference of the factory wheel/tires combination. I do not know if there are tire sizes for the 19" wheels that would bring the numbers back into line. You would have to visit say TireRack and see what 19" tire sizes are available and plug in the numbers in the tire diameter calculator and select front and rear tire sizes that result in revs/mile numbers the have the rear tires making a bit more revs/mile than the fronts and at around the same +0.49 ratio.

Interestingly, the winter tire set recommended by Porsche is 225/40-18 and 265/35-18. Those are 829 (front) and 822 (rear) revs per mile when new. That is -0.85%. If the fronts are half used and the rears are new, the difference increases to -2%.

225/40 r18 265/35 r18 is also standard on 996 c4 1999-2001. I tried to figure out how the AWD really works and how and when the figures 5-40% torque transfer to front applies as well as the impact of tire size and tire brand and rim width on it since I own the car (2001). I'm still not 100% certain today. http://www.porsche.com/usa/accessori.../tyreapproval/

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oops. I meant 45 degrees. I got my information from here http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=273 ive driven mps in snow before too. I didn't get stuck but now I'm not a stupid 19 year old. I'm a slightly less stupid 30 year old haha.

Thanks for all of the info everyone. I'm going to do some digging into the existing threads as well - and I've reached out to the dealer to see if they can provide any insights. One thing that has me a bit stumped. I believe these wheels are either standard or optional equipment on the 997 C4S. From what I found online the tire size they shipped with was the same as I have in the front, but they went with 305 in the rear. That makes the rears even larger and moves the variation in circumference to 2.7%. Maybe I'm missing something, but if they came from the factory with that setup, I'd conclude that means it is acceptable.

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Edit: Yes the 2008 C4S came that way with viscous system...the AWD system changed in MY09 ...interesting.

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The 2005-2008 C4 and C4S have a different final drive ratio for the front, which allows them to use larger tires in the rear...

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I thought the drive ratios where the same front/rear except for the Tip S, see page 259 of Porsche 996 The Essential Companion Supreme Porsche.

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Yes, the winter tire recomendation from Porsche for the 996 C4S is 225/40-18 and 265/35-18. Using a tire size calcuator at: https://tiresize.com/calculator/ The revs per mile is 804 for the fronts and 797 for the rears. In this case the speed difference of the front and rear tires is not only reversed from the summer tires but about double the ratio (-0.89 vs. +0.49). All I can offer is this: The above is for winter tires and these are not rated to go above 240/kmh (149mph). I dare hazard a guess it would be rare for one driving on true winter tires to drive even at half that speed. (I've been on snowy roads and in two notable cases on a major freeway too and under the conditions a reasonable max. speed was in the 45mph range or slower. (Slower in at least one case when road conditions were so bad some vehicles had chains.) In my experience while a number of vehicles were traveling faster in the passing lane there were a number of accidents. In one case I managed to get off the freeway before it was closed due ot accidents. In another case I pressed on and managed to make it through the snowy section -- total distance around 50 miles -- without encountering a road closure. But I was lucky for I did spot a number of vehicles -- passenger vehicles and big rigs -- off the road. Heck even a tow truck driver managed to lose his rig in the snow.) While I would not have a problem running a set of winter tires such as the above on my Turbo I would not run a set of summer tires with the mixed up front and rear revs/mile and at any ratio too far removed from the "+0.49" ratio provided for by the factor sanctioned summer wheel/tire sets.

Tire size calculator does not seem specific enough, dimensions differ across tire brands for same sizes. Looking at the Porsche link I posted above, I wondered in the past why the PS2 was not approved for a 2000 C4 whilst it was for a 2000 C2 (same size). In the latest version of pdf as seen now through the link, PS2 was removed from the C2 too.. Checking the Rosso's based on data published by Pirelli showed the rear being smaller, same for the S02a. PS2 was different, rear taller than front. (all 225/40 front, 265/35 rear, on standard Porsche Rim width) I asked the dealer service dep, the gold certified mech, wrote to Porsche, never received an answer which would wrap the whole thing with every bit making sense and let me satisfied with the answer. I still wonder what the effect on the AWD is when the fronts are smaller than rear. Winter is yet another animal, I run 205/50 front and 225/45 rear, whilst not on the approved tire pdf, these dimensions are in my owner manual, and whilst skinny, they are a true pleasure in the snow (I live high up in the alps during winter). Currently on snowsport 240 as recommended up to last year but need to replace and well can't find anything where the rear would be smaller.

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I have a '01 C4 and came with 285/30/18 rears. Strange how Porsche has the same spec turbo twist wheel for the '01 and '02 cars, but spec'd 265/35 in '01 and 285/30 in '02. Both are narrow body cars.

Just wanted to post a quick update. I'll post more details when I have some time. I heard back from 1 dealership so far and they said that the max acceptable difference is 3%, but ideal is as close as possible to a match. They pretty much said I'm right at the limit and its up to me how to proceed - essentially if I screw it up, its on me. I did some research and found someone on the Subaru forum that measured how much diameter changes based on each additional PSI of inflation. Its pretty linear above 32 PSI, however I believe its specific to the tire size. Still, I used their findings in a formula and determined that with my fronts at 40 and rear at 32, that puts my variation at only 0.9% even when factoring the front tread wear into account. I know as someone posted earlier that this may not be an ideal solution - BUT I wanted to do it for my own peace of mind for testing. The results: I noticed a difference in the first 10 seconds of backing out of the driveway, and immediately when driving down the street. The car feels back to how it did before and is no longer straining. I've seen some argue that tire pressure has no affect on circumference or rolling circumference and I disagree. Not only do I believe adjusting the inflation made a noticeable difference, but some tire pressure monitoring systems work completely off wheel speed to determine when a tire is low on air. So, that's all of the news for now. I'm still not sure what my next step will be - but I'm happy that at the moment I'm not tearing up the clutch. Cheers Charlie

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What's your sticker on the car say for recommended pressure 36F/44R? Your going to make handling worse...

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Yes, except 2005-2008 is the 997.

The tire pressure experiment is interesting but those pressures seem way too far wrong to be a good idea. I still suggest that you replace the front tires with 225/40-19s. Since the existing tires are partially warn, the cost of the new tires is not a complete waste.

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Thanks for the correction. Confused with the OPs car.

Thanks for the input everyone. I agree that the current tire pressure ratios aren't ideal - but for my experimentation they've served a purpose. Since there really doesn't appear to be a lot of concrete info on this I'll post some of the things I've confirmed. Running 295/30/19 in rear and 235/35/19 in the front appears to work fine with no major problems related to the coupling. I cannot comment on how it impacts the effectiveness of the AWD system as I've never had the proper stock setup to compare. I can confirm that when I had the car on jack stands and put it in gear, the front tires turned immediately. So that leads me to believe that the tire setup above hasn't destroyed the coupling. With even tire pressure and tread wear the variation is about 1.9% Replacing the rear tires, but not the front, put things out of spec and caused the coupling to engage and resist the car moving forward. At that time front tire pressure was 34 and rear was 40.5. With that tire pressure, and factoring wear of the front tires into account it put the circumference variation at approximately 3.5%. That was not good. Based on the same estimations of change based on tire pressure adjustments I brought the variation to .9% (40psi front and 32 rear) and everything seems back to normal. I'm going to play with the numbers a bit more and do some research to see if 235/40s would fit in the front before I make a decision on what to do next. Cheers