papa - Did you care about the wheel size or the tire size? Because you asked about wheels which are the metal part, now you are talking about the rubber part...
Anyway, I found my 1997 Gulf GTR press kit and the wheel dimensions listed within are indeed a match for the sizes already mentioned. I'll look for secondary confirmation of that from another source as well. There is no measurement supplied for the tires.
Their slick tires would not be sized like a traditional road tire - the sidewall measurement of a slick is not a ratio of the width, but rather their actual measure of their height in millimeters.
>8^)
ER