255/35/18 and you want tires that tend to run wide like AD08r's. I am assuming you already have the mods for these to fit because they DO NOT fit a stock bodied S2000. You need wider fenders front and rear.

 

On a 10" wide wheel, the 255 in AD08r will have a slight stretch. 275 in AD08r will be nearly square on the wheel because they run wider than other tires, even wider than 295 Michelin Super Sports.

As far as these properly fitting the S2000 that is another story.

 

I run 255/40/17 AD08r's on my 17x10 +35's and they are pretty square. Maybe just the tiniest bit of stretch if you could even call it that. The issue with 275 is they wind up being taller. The 17 inch ad08r's have a 10 inch wide tread width. so it winds up being fairly square but maybe it is a little different on an 18 inch wheel with the shorter tire sidewall.

 

 

Cool. Post pictures when you get everything done!

 

[left]will do!

 

 

A 265 or a 275 will be a better fit on a 10" wheel. 35 aspect ratio is what you'll want.