RouvalisC63

Hello I just bought a pair of Forgestar F14's for my 09 c63 amg.

Fronts: 18x8.5 ET 42
Rears: 18x9.5 ET 55

Would it be ok to run them on the stock tire size? I spoke to a sales rep on tire rack he seemed to think they would strech a bit but nothing major.
Fronts: 235/40/18
Rears: 255/35/18

Bramage

I was just looking at some Nitto NT05 tires, and they show recommended wheel width based on tire size. There are also a number of calculators online, but I think this is a good start

They don't show a 255/35, however the tires you specify appear to be within recommendations. You should be ok with a little stretch on the back

Size-----------Load-----Stk#--Tread--Diam--Width--Wheel Min-(Rec)-Max
*235/40ZR18 95W207-160 7.2 25.43 9.57 8.0-(8.5)-9.5*
265/35ZR18 97W207-060 7.2 25.31 10.59 9.0-(9.5)-10.5

CampioniShop

Slight stretch - but pretty equal all around. Ideally you want a 245 up front and 265 at the rear.

Let us know once you receive the wheels