tire sidewall make a difference

I was just reading the manual for a C4 and it says that the rear tire sidewall should be 35R and the front 40R. I did not see this and got 35R all the way round. What effect is this going to have?

probly just a slightly harsher ride and more likely to get dented rim.

Ok so this will have no effect on the mechanics/ axels etc? And i was wondering what the reason is for having 35 on the rear and 40 at the front.

by mounting these tire you will "deactivate" the 4wd due to the lack of rotational difference front to rear.

oekodriver - deactivate the 4wd! What have you been smoking? Care4era - The 35R and 40R are relative to the width of the tire. Many people will run a lower profile with wider width to avoid clearance problems. Search some of the threads. You will see many people changing from 265/35's on the rear to 275/30 and 285/30.

care4era, the lower profile tires "may" give you a smaller tire diameter. I say "may" because it's relative to the tire width. Just check the tire specs, and you shoud be able to figure out the diameter relative to the recommended sizes. The AWD system has a viscous coupling, and power delivery is adjusted based on rotational differences between front and rear tires. If you don't maintain the recommended tire sizes and change the rotational difference between front and rear tires, the viscous coupling may not function as designed. This will be further compounded if you have PSM and ABS, two other systems that depend on the rotaional differences between the tires and their respective axles. Having said that, whether this theoretical change is significant in the real world or not, I couldn't tell you. I've never challenged the recommendations, and have tried to stay relatively close to the diameters of the recommended tires.

What are the exact sizes of your wheels and tires?

You see alot of owners running non-standard size tires. I have looked at the relative sidewall heights as follows: Standard 225/40=90 265/35=92.75 235/40=94 265/40=106 Many recommend 275/30 and 285/30 on the rear 225/40=90 275/30=82.5 235/40=94 275/30=82.5 225/40=90 285/30=85.5 235/40=94 285/30=85.5 With PSM and 4wd the concern is relative size. You can see that There is a wide variation of the delta in the sizes you see in postings here and on Rennlist. I believe these are all milimeters. The stock is only about 3mm, but you can see there are alot of owners running larger deltas. Anybody out there running 275 or 285's on the rear and the standard 225 or even a 235 on the front? have you had problems?

Tire size is only part of the equation. You need to factor in the size of the wheel too, as the tire's overall dimensions will vary slightly. http://www.1010tires.com/

Ok so reccommended is rear 265/35R 18 I have 265/35R 18 front 225/40R 18 I have front 225/35R 18 So as you can see the only difference is the sidewall on the front tires. Spec says use 40 and I used 35

What is the width of your front wheel? 8"? 7.5"? 8.5"?

heh, that i cannot tell you. I just know they are 18"

@Dharn55 you might want to check your knowledge about the c4 and its viscous coupling. from design the rear wheel/tire combo diameter should be smaller than front. this achieves the permanent ca 5% torque on the front. If now suddenly you mount a same size or smaller diameter front wheel/tire combo than on the rear you effectively "disable" the intended effect of the viscous coupling thus 4wd. Of course it will still work in case of massive slip of the rear but not in the way it was designed for. and btw you should smoke one more.. maybe it helps @care4era you should get a set of 225/40 18 for the front

I would agree with this too.

That means your front tire/wheel rolling diameter is smaller, and therefore rotates faster than the recommended 40 series. Not up to specs, and theoretically boogers up the viscous coupling and ABS, and performance. I'd change to a 40 series.

On the awd Porsches, the tire diameters are pretty critical. Rather than find out the hard way, I would personally buy the correct tires. That may sound wasteful, but wait to see the bill to repair any problems. That will be wasteful. AS

According to Tirerack there are two OEM tire combos for a bone stock '02 C4S: - Pirelli Assymetricos - Continental Sport Contact 2 Of these two ostensibly Porsche-blessed combos one (Pirelli) has a smaller (slightly) rear overall diameter and the other (Conti) has a larger (slightly) rear overall diameter. This makes me tend to believe that the Porsche viscous AWD design is more likely intended to have equal front/rear overall wheel/tire combo diameters, but, as with any engineering design, there is a small amount of slop to either side of "ideal" which won't really have much affect on the viscous system maintaining its 5% front torque. Do I have too much time on my hands or what?

I will never buy anything from Tirerack ever again, and neither should anyone else. As for the OEM tires, I am pretty sure Michelin Pilot Sports are offered in "N" sizes for this car.

well, I won't judge tirerack, but their data is wrong. from the source: http://www.conti-online.com/generato...an_2005_de.pdf a 225/40 18 n2 sport contact2 has a circumference of 1945 whilst a 295/30 18 n2 sc2 one of 1935 the difference is minimal but the rear is smaller. anyway.. I think Kirk S is right on, diameter front/right should be within 3%.

heh, some say I should, some say the effect will be negligable. Who to believe

Whomever you had install the wrong size should swap with the correct size with no expense to you.

You checked the manual and found the recommended tire size. The you went out and bought a different size. Now, after you bought them you start asking yourself if it was really a good idea to go that route. So what do you want to hear? Yes, I think Porsche had something in mind when they made up the tire recommendations. No, I don't think you should divert from these recommendations if you don't have a good reason to do so. Which you obviously don't have. No offense - just my 2 cents Thorsten

Thorsten, i am going to change on Monday. Just did not want to do it needlesly.

What year C4 are you talking about?

OK guys, the standard tires on a 00 C4 Cab are front 225/40/18 and rear 265/35/18. So if you look at my previous post, the factory set up has a larger diameter on the rear, not on the front. So from the factory the fronts are turning faster than the rears, not vice versa. I have always run the factory sizes, but I am thinking of going to 265/40/18 and 235/40/18, where the difference between the front and rear diameter is even greater. However, you see alot of owners going to 275's and 285's on the rear, as discussed in this site.If they go to a 30 series tires, then the rears become a smaller diameter than the fronts. If they stay with a 35 series tire, the rears are still a larger diameter than the fronts. Anybody out there running a 275 or 285 tire?

"What year C4 are you talking about?" 2004

If you do that, your rear wheel diameter will be 3.95% larger than stock, which is NOT recommended (a whole inch bigger). Going to a 285/35 however, results in a 2.13% increase, which IS within reasonable margins, but you're looking at a slight degradation of performance in the acceleration (countered by a slightly higher top speed). http://www.1010tires.com/TireSizeCalculator.asp

I got the specs for Michelin PS2 as follows: Size Revs/mile 225/40/18 829 235/40/18 819 265/35/18 822 265/40/18 790 275/35/18 814 275/40/18 783 So on the stock set up for a 2000 C4 the the front are smaller and are turning 7 less revs per mile, not huge, but it is there. However, it is the opposite of what you said (care4era). I am a little concerned that going to a 235/40 and 265/40 set up increases this differential to 29 revs per mile.

Just noticed tirerack's "allegedly" smaller diameter conti sport contact 2 (the front one) has a slightly lower revs/mile than the allegedly slightly larger diameter tire (comparing the same two oem conti sport 2 tires for an '02 C4S). But a smaller diameter tire should spin more times, not less times, in a mile. So, lesson learned, NEVER get your information from only one source tirerack = bad data

As you get into later year C4's (03 and 04) the rears go to 275 and 285 in a 30 seires tire, so the rears do become a smaller diameter. Did Porsche change the viscous coupling? When I look in my PET program, there is only one front drive system listed for all cars (996's) from 99 on, so it does not appear that they did change the drive system. I can believe that Porsche may have reprogrammed the PSM though.

Assuming its not an S, you should be running a 225/40 and 285/30 setup.

ps2 265 225 was never a stock setup. not "n" tires anyway. (anyway best thing out there imho, what I run) conti sc and pirelli were stock and both are the same exact size of the ps2. (diameter) 25.1 front 25.3 rear on a 7.5 wheel front, and a 10 rear, which results in a bingo !! smaller !! diameter on the rear. 7.5 beeing relatively narrow pushing/squeezing the tire more vertically out. the 10 beeing on upper limit for a 265, flattens the rubber. also take the rapid wear of the rear compared to the front into account wich will accentuate this minimal difference rapidly. to the original poster. justin is right, if your car is a 2004 c4 225/40 285/30 would be stock for summer. 225/40 265/35 is proposed for winter rubber

I hate to be argumentative, but I really don't think a narrow wheel can change the diameter of a tire, hard to strech those steel belts. I agree that as the rears wear they would become slightly smaller. However, we still see that on early C4's the rears started as a larger diameter, and on later years the rears started as a smaller diameter.