When you say "the smallest size possible", do you mean height or width ? Also, what kind of tires are you looking for ? Drag radials, slicks, all seaons ?
I am extremely satisfied with my 275/35R18 Nitto NT555s. I just got them put on today with my brand new 18x9.5" Y2K Cobra Rs.
With the right amount of negative camber in the front (I have about 1.0 degree), the wheels/tires won't even stick out.
It really does matter what brand you buy though. I paid more for the Nittos because I didn't want to buy these wheels and half-@$$ myself on the tires.
The best fit for the Y2K wheels are a 285-35-18 or 285-40-18. This is based on the width of the wheel and the mounting of the tire. A 275 will work but the sidewalls will bow outward a bit, which is not condusive to optimum tire performance. With the 285's the sidewalls will be parallel to the wheel rim face and look flat. That's what a proper tire on the proper wheel should look like technically. With Stangs, you have to be carefull and wise in your choice of wheels/tires...because the wrong choice (and it is soooo easy to make the wrong choice) will make the wheel/tire combo stick outside the fenderline. Check out the stock Y2K Cobra R...look at the front end straight on with the stock wheel/tire combo...they stick out a tad..and that's what you get with most 9 to 9.5" wheels. Been there done that...and Now I have custom off set Halibrand wheels..AWESOME!!! Do a search on www.mach1registry.com for Team III Wheels...