I acquired a set of 17" Fikse FM 10's for sumer wheels for my NB C4. The wheels came with a new set of Continental Contact Extremes, but the tire sizes don't seem to jibe between the fronts and rears...

The wheels are 8" wide in the front, and 10" in the rear. The fronts are running 225x45x17, but the rears are shod with 275x40x17. It seems to me that the rears are too big circumferentially to run in conjunction with the fronts, and that it will wreak havoc on the four wheel drive and possibly the ABS? And if so, is this only a problem on the 4 models, or are these tires simply mismatched altogether? I think the proper size is 255x40x17 in the rear and that I need to ditch the 275's. Can someone please confirm this?

Thanks in advance, and sorry if this question has been asked before...





Fikse FM 10's with new Continental Tires





275x40x17 rears





225x45x17 fronts

 

Great looking wheels Foxman. The difference in diameter between them is only .6" Not sure if that is a significant difference for the C4. Having a 4 myself, I'd go with the 255 to be sure, same Conti.

 

5% is the magic number where there are issues with ABS, I believe this is the same for the AWD system (correct me if wrong...)

The difference between stock tires on these cars front to rear are approx 0.1" on 17" and 0.3" on the 18" for the C4 on the C4S and Turbo the difference was around 0.5".

I really don't see and issue with what you have,

 

225/45/17= overall diameter 24.97
275/40/17 = overall diameter 25.66
difference %2.76

255/50/17 = overall diameter 25.03
difference %0.24

the 255's are a better match in terms of overall diameter, but they will look different, a 255 on a 10" wide wheel will look stretched compared to your front wheel.

i personally run 225/40/18 and 275/35/18 and have never encountered any issues with ABS or ABD traction control lights coming on during normal driving.
my difference %1.96
i remember reading that the threshold front/back on a C4 from Porsche was 3-4%
hopefully someone can either confirm that or give the correct %

also here is a list of Porsche approved sizes:
http://www.pcarworkshop.com/images/0...1997_64047.pdf

 

Thanks for the replies - all really helpful! I better understand the calculations now and crunched the numbers and came very close to Bigrig's calculations with a 2.69% variance. I'm told the maximum variance for the C4 is 3%, so I appear to be just within the parameters and good to go!

I knew something was amiss when the shipper insisted the rear tires needed a wider box - really pleased to know I don't need to buy another pair of tires! Cheers!

 

Sweet rims! They must be so light!
Can you post a close-up pic of the rears installed on the car?
17x10 must look great! What offset are they?

 

Wheels just shipped out today so I will have to report back to you on the offsets and a close up. In the meantime here's a picture of them on the PO's former 993. Looks like I need to sources some "Fikse" caps.