I don't know what you mean by "thickness" but 265/25.4 = 10.433" which is the width and 10.433 x .35 = 3.65" which is the sidewall height. If your C class and your Porsche both have 265 tires, they are both 10.433" wide (within an 1/8").Sounds great! If I may ask, what is the tire thickness with a 275 (in inches)? Did you change wheels for aesthetics or to fit wider track?
As I know the standard car has 265/35 rear wheels with a tire width of approx 8.5inches. The reason I ask is that my C-class has 265 rear with tire width measured at 11 inches.