03 C4S Front/ Rear Tire Revolutions / Mile

I am looking at winter tires for my C4S and I already have a set of Continental Extream Contacts DWS06 with less than 2500 miles on them but I am worried about their sizing. Fronts are 235/40 18 Revs/ Mile 818 Rears are 295/35 18 Revs/ Mile 796 I think I read about AWD cars needing to be with-in 10 revs/mile, thoughts? I could buy a set of new fronts at 235/45 18 and get with-in the 10 revs.....

If you want the AWD system to work property, you need to have the front tires larger than the rear. You have them the other way around. Been there, tried that.

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Wrong! The standard tires for C4's and C4S' have more revs per mile on the front than the rear. It has always been this way.

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Well, not ALWAYS. For the record, this configuration is definitely not within +/- 10 revs per mile...

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For the Pilot Sport 2 225/40R18 front = 829 Revs per mile 295/30r18 rear = 832 Revs per mile 285/30r18 rear = 840 revs per mile So...you were saying?

Another option would be to get new rears in 275/35-18 and keep the 235/40-18 fronts you have. The 235/45-18 fronts could be tight and rub on the fender liner. Just to be controversial, I have 235/40-18 and 285/35-18 winter tires and they seem fine to me. I have heard of people who have 235/40-18 and 295/35-18 tires and were ok.

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You're right, the fronts are larger than the rears. The rears rotate faster hence more rev/mile providing the preload which continuously provides positive drive to the front axle.

I would ignore the concerns about keeping the front taller. The Porsche summer recommended tire combination is slightly taller (fewer revs) front than rear, but the Porsche recommended winter tire size has a slightly taller rear tire. Both sizes are recommended by Porsche, and both are within 1% of each other. That is the crucial metric on sizing for the AWD. Any variation in wheel speed front to rear will heat the fluid in the viscous coupling and apply some load to the front wheels. The system is designed to have a small amount of load delivered under higher speed normal driving conditions. The key is not to apply too much load in regular driving, caused by having too great a discrepancy between front and rear. Again, if you're within a reasonable percentage (Porsche sizing is just less than 1% difference) you're not harming the system. As for your proposed sizing, at just less than 3% I don't know if that's within the system's tolerance or not, but I would go with a 275/35 18 on the rear, that'll get you within 1%, and provide a narrower rear which is what you want for better snow traction anyway. A 275 is hardly ideal on snow, let alone a 295. I run 235/40 18 and 275/35 18 on my TT in the winter, and I can tell you that 1) a Porsche tech recommended this sizing, 2) I have not experienced any damage to my awd system, 3) and the system functions properly (i.e. when the back wheels slip, the front tires are given power, you can feel it at work with PSM off when sliding around).

I bought a set of 17" Boxster wheels for my 2002 C4 for winter tires (17x7, 17x8.5). They fit fine (offset is ok) and don't look too bad. Anything looks better than a Porsche in the ditch. The best winter setup I have found is Front 205/50/17 - 25.1" (+0.4%) Back 225/45/17 - 25.0" They are narrow, and the fronts slightly larger than the rears which is ideal. I get excellent traction. Go to Whistler on some the snowiest days and have no problem. Not sure if this would work on a C4S.

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Just to add my 2 cents... Summer tires for these cars want to be sized so the front wheels/tires make fewer revs/per mile than the rear wheels/tires. IOWs, the rear wheels/tires should make a bit more revs per mile than the front wheels/tires in order to heat the special fluid in the viscous coupling so torque is transferred from the rear axle to the front axle. The minimum torque number quoted by Porsche is 5%. There is no speed specified at which this low number occurs. The maximum torque number is 40% which is said to occur at around the 150mph mark. If one does the "math" with Porsche sanctioned wheels/summer tires sets he'll find the above true. The rear wheels/tires make more revs/mile than the fronts. Not that many more revs per mile though. OTOH, if one does the "math" for Porsche sanctioned wheels/winter tire sets he'll find the above is not true. The front wheels/tires can make more revs per mile than the rear wheels/tires. In this case one "loses" the benefits of the AWD system but frankly I'm not sure just how much it would help. The biggest factor in winter driving is having proper winter tires fitted. It is tires that make the biggest impact not the AWD feature even if one managed to fit a set of winter tires that had the desired revs/mile ratio. The AWD system is not intended to pull the car out of a situation that has the rear wheels/tires spinning. All this can do is ruin the viscous coupling/front diff in which it resides.