GVA-SFO

I would be interested to know what is the factory specs about the Tires Sets for C4/C4S.

I had a dealer installation on a C4, i.e.: on 18" rims and they are:
Front : 225 x 40
Rear : 285 x 30
It took some time to realize, but I'm thinking now that this may be very wrong, as :
225 x .4 = 90 while 285 x .30 = 85.50
..or about 5% in the circumference size !
This should be quite bad for the center differential !!

Do anybody know what is the factory specs for the circumference difference in % ?

balefire

the tire sizes are the correct OEM sizes for a narrow body C4 on 18x10 rear wheels.
the difference in circumference is 1.4%
your math is wrong.
you need to account for the wheel diameter as well.
this may help you.
http://www.miata.net/garage/tirecalc.html

Zookie

wrong Rear size for C4S... correct size for C4

C4CRNA

C4 is narrow body
C4S is wide body

Yomi

C4S is wide body. For the C4S:

Stock front is 18x8 ET 50 5x130 with 225/40 ZR 18 N3 tire.
Stock rear is 18x11 ET 45 5x130 with 295/30 ZR 18 N3 tire.
[The XRC option, Sport Techno wheel, uses ET 63 in the rear with a 17mm spacer]

I believe the rolling diameters for my car stock were 25.2" front and 25" rear. That's a bit less than 1% difference. The Advan AD08's in the same sizes both claim 25.0" so in theory they're the same rolling diameter.

You can find lots of threads about it, with no definite answer. The more difference, the shorter lifetime the viscous center diff will have (with lots of caveats). Most people say to stay within 4%, which is a pretty big difference. My personal preference based on nothing but reading threads and gut feel is to stay within 2%.

GVA-SFO

Thank you very much for the very good answers.

And, thanks for the (right) point on my wrong math.
So, I'm on about 1.4% difference, and these are the OEM sizes.
I'm happy to now know that.

Fyi, I tried (I still have a set of turbo twist hollow somewhere in the garage !) the 11", ET45, but, it is true that they stick out on the narrow body. This is why, I went to the "funny" "turbo twist non hollow" 10, ET65, that I do not think they are OEM at all, but, at least, they do fit the the rear nicely.

(In the front, I noted that for both narrow and wide body, rims are the same, i.e.: 8", ET50.)

(Originally, I was on 17" rims and never asked myself that question, as I kept putting the same tires on, but got a 2nd set, i.e.: "turbo twist" and very recently had this doubt !)

Btw, I was very careful on this matter with the Cayenne (06 CS Tit Ed.), as instead of running on the OEM set (on the OEM Techno's, 20 rims are 9" on front and 10" on rear) and came from new with all four on 275, I'm now on the second set staggered, i.e.: 275/40 on front and 315/35 on rear, and with those, the diff is pretty close to 0%.
(For info, when towing, i.e.: 6'000 pounds, the 315 on the rear imo, do improve the lateral stability.)

PS: Funny to know that on the Lambo Diablo VT, the front wheels are smaller but the center differential is still happy as a gears ratio in the front do the adapting job !

Dennis C

Not all of them were staggered! I have OEM 20" Sport Technos straight from the factory on my Cayenne and they are 9" all the way around.

GVA-SFO

Hello Dennis,
I heard this already (we are a bit out of topic, but..).
I think it is a question of manuf. years, and in 06, they started to have Techno's in 9 and 10.

I bought a 5th wheel, (not an OEM, a cheapo knock-off), that is a 9" and mounted the best of old 275/40 on it. Some time, I take it with us as spare (but not often, ..as it take a lot of space in the back !)

brad@tirerack.com

GVA, please call me if I can help.

kr3678

FYI, my 1999 Carrera 4 has 265/35 18 on 18x10 wheels as the OEM size in the rear.