RedCelica

Then offset, fender modification, how much you're lowered, and what you plan to do with the car (DD [climate considerations]/weekend warrior/autox/track etc) all need to be factored in for us to give you an optimum answer.

ZDan

What is optimal will depend on your specific car setup (horsepower, suspension settings, etc), usage (slow or fast autoX, medium or high-speed road courses, street, etc.), specific tire make/model, and to some extent on your specific driving style and preferences. It is not a foregone conclusion what the best tire size will be, wider isn't always better, and neither is running the narrowest allowable tire size for a given wheel width.

Janus00

I run 245/35's on my 9.5" fronts. Widest is not always best. During hard cornering you don't want the sidewall to flex. I'm lowered 10mm, and only am doing street driving, but still enjoy the stiffness from the slightly stretched sidewalls.

1nate7

You might find this useful, pics of 18 x 9.5" +45 RPF1 with 265/35:
https://www.s2ki.com/forums/s2000-ga...ekend-1121364/

I would not go any wider than 265/35 and would probably even stick with 255/35 depending on tire availability.

denlin68

265/35 is pretty much the widest

THMotorsports

255 or 265 is a great size for a 9.5. 275 is pretty square. Anything wider than that is pretty pinched. You'll notice a sloppy feeling, and it won't give a great contact patch either.

Though it depends somewhat on the tire, some tires run MUCH wider than others. I had a set of 245 Dunlop SP600s that were about 15mm narrower than 245 Dunlop Direzza Z1 Starspecs. And the 245 Michelin PS2 was even narrower.

s2000Junky

Yep. My 275/40/17 NT05 are the same width as my previous 255 Kuhmo XS. Making fitment on my 10" pretty perfect. Sidewall height is still different of course. The XS run wider and the NT05 run narrow, apparently meeting in the same place, but two pretty dramatically different sizes. Even the same 200TW category tires. Breaking all the typical rules here. So always one fitment exception out there it seems. Makes it difficult to know how to size/optimize things sometimes without direct experience from other members.