f4 plt

A question I'm sure is not asked often. What is the diameter of the wheel tire on an Cayenne S? I know the size is 255/R18. My reason in asking this is that we bought the GTS primarily for the seats (bad back) but I would like to put a smaller wheel with tires having slightly higher sidewalls for normal driving. The 21" wheels can easily be damaged by pot holes that are common around here. Comments and or suggestions please

User 4221

I posted this on another site so please ignore some of the non-essential info and different tire sizes............

What really matters is the circumference of the tires

Stock circumference: (225/45-17)
Width: 225mm
Aspect Ratio: 45
Sidewall height: (225 x .45) = 101.25mm
Convert to inches: (25.4 mm/inch) = 101.25mm/25.4mm/inch = 3.986 inches
Wheel diameter: 17 inches
Overall diameter (17 inches + 3.986 inches + 3.986 inches) = 24.97 inches
Circumference is (Pi * diameter) = 3.14159*24.97inches = 78.45 inches

New circumference: (225/35-20)..... I'll skip the annotations and do the direct math:
Sidewall Height: 225 x 0.3 = 67.5mm or 2.66 inches
Wheel diameter: 20 inches
Overall diameter: 25.3 inches
Circumference: 79.52 inches

The new tire/wheel setup is 1.07 inches bigger in circumference. This means you will be going faster than indicated on the speedo. The approximate change is (1.07 inches/78.45 inches) = 1.36%

Meaning, at an indicated speed of 60 MPH, you will actually be traveling 60.8 MPH.

This not a big delta as most cops give 5-10% in speed for differences in tire sizes from mfr to mfr.
The best way to verify is to get a GPS and compare against the speedo.

bgsntth

Hi f4,

I'm with you on all counts, and have been researching 19" wheels for my '09 GTS. Apparently (lots of contradictory information), 18" wheels will not fit on the '09 GTS due to the calipers getting bigger compared to the '08. Hence, I'm looking for 19".

For 19", the correct size is 275/45-19. I'm going to most likely go with a 265/50-19 Pirelli Assimmetro, which is about .4" bigger in Diameter, as I am planning on some off-tarmac driving. Right now I'm leaning toward the Victor Equipment Zehn Wheel, which Suncoastparts says will fit over the calipers. Next pick would be the Porshe Sport Design wheel, which is readily available on ebay. Miata.net and tires.com have really good wheel calculators if you want to play around with tire sizes.

romeroe

Hi F4 Plt,
I have a Cayenne S with the 18s. If you want to trade wheels & tires, just let me know.

romeroe

Oops, I should have read bgsnntth's reply and in that case, please disregard. Good luck.

JHoffman

Scott C,
Great part about cars w/ tpms is that you select the tire size in you onboard computer you currently have on the vehicle. This adapts the dash for the correct tire pressure and the speedo for tire size.