SHELBY GT 350 Member
Joined: Jan 2011
Location: Central Massachusetts
255/40-20 tire is about 28" tall (outer diameter); 285/35-20 is just a hair shorter. That is about 1" taller than the "normal" 27.2", assuming the normal has not changed with the S550 cars. That is only about 1/2" on the radius and I believe it should clear with no issues, assuming the car is not extremely lowered.
My experience is also with the S197, hope someone with direct S550 experience will chime in.
| 2010 GT coupe | Daily Driver with some Steeda and GT500 take-off stuff for the occasional track day