Q&A

Goedenavond,

Every now and then a good set of used Porsche Cup Tires comes available.
How do these sizes relate to road tires. How do i convert the sizes ?

For example:
http://cgi.ebay.de/Porsche-CUP-Slick...#ht_500wt_1154

Would this set: 26/68-18 & 24/64-18 fit on my original 996 GT3 MKII wheels: 235/40/18 & 295/30/18 ?

911SLOW

example
road tyre 235/40/18 *std front tyre of gt3 2004
means
235mm width
the 40 is percentage of the width of the tyre representing the tyre wall
and 18 is wheel diameter in inches
to find the full diameter of tyre and wheel
you add 18x2,45= 45,72cm plus 23,5cm x 40%= 9,4 cm for each side
45,72 + 9,4 + 9,4= 64,52 cm

slick numbers are
24/64-18
24 width in cm
64 overall diameter in cm
18 wheel size

Do the math

JOhn

himself

FWIW, I can't fit any 68 on my ride. 64 fits just fine though.

-td

spare tire

The fronts fit and work just fine, but the rears will not work at all and fitting under the fenders is the least of your worries. The 68 in the tire designation is the culprit. They are so much larger than the fronts that your ABS goes into full meltdown, lights flash, the front wheels lock up when you try to brake and the brake bias is way off. On a street GT3 do not waist time with cup tires. If you can find slicks that have the same size front and rear then enjoy them.

va122

What he said

jrgordonsenior

I occasionally run the cup sized 680's in the rear in my 996 and have plenty of room to spare. What makes the GT3's different?

911SLOW

The 680 is 4,5cm taller than the oem tyre. Its even taller than running 305/30/19 which are 66,5cm tall.
With regular height maybe it will clear the linings but for sure if you mix it up with a 64cm tall front tyre you are looking for ABS trouble on the street car.

John

himself

Not sure what the issue was, but 305/680 on my car was touching the fender. Maybe was a rim offset issue.

[edit: I see that the cup sizes are 28/68, that might fit - but never tried]

-td

roberga

The tirres are fine IF your car is set up to take advantage. springs, brakes and such.

va122

not true. The abs will go crazy. It's happened to me when hoosier sent me cup size slicks vs the 645s

spare tire

I am giving first hand advise. I put cup blue stickers on my street GT3. The fit was fine. The car had nice down hill rake to it. ( due to the taller rear tires) I live out in farmland and I fired her up to 90 mph and tried to brake hard and the ABS went nuts. The front tire locked up very early for the pedal effort and stayed locked up and an ABS FAILURE idiot light came on and stayed on. I turned around and went home and tried to brake a few more times and the fronts locked up every time, no ABS. So it has been done and I waisted $100.00 mounting tires that wont work and I bought the used tires also. Even IF you disabled the ABS and the ABS light you would still have a big problem with bias.
The fronts lock before you get any help from the rear brakes. So your stopping distances would increase by an unacceptable margin. I then got ahold of some
27-65-18s off of a friends 2002 GT3R (his front tire) and put that on the rear. I corded it and need alignment to get maximum use.

jrgordonsenior

It seems the GT3's are more prone going into "ice mode" than the 996's. We have 4 or 5 99'-02' 996 race cars running the Yokohama Patron cup tires down here without any ABS issues. Weird huh?....

spare tire

It is all part of the 996 gt3 magic.

Q&A

Thanks guys but it seems more of a hassle than i hoped for. I'll stick to the michelin cup sports for now.

Q&A

I also found this link in the mean time. Could come in handy:

http://www.rennreifen.at/listReifen-...rke=6&Bewerb=1