Kumho SPT street tires
It's finally spring time and I've been waiting to get some new tires to replace the garage-aged tires that came with my used Prodrive wheels. I've done my research over the winter and in the end due to various reasons I decided to try out the relatively new-kid-on-the-block. I've always had positive experience with Kumho tires, including 1 set of 712 street tires and 4 sets of Ecsta V700 R-compounds plus experience driving a few other sets of R-compounds on other cars and I can't wait to test out the new tires.
I know many people would go for 225/45/17 and 255/40/17 for the wheel widths I run (7.5 and 8.5). While people like to run max tire width for a given wheel width, I like to run max wheel width for a given tire. With my max-wheel theory in mind I didn't settle for anything narrower than 7.5/8.5. Already have tire-size in mind when I went for the wheels, I got the
215/45/17 
and 245/40/17. 
When I went to pick up my tires, my first impression was "Wow"! The 245s are definitely wider than the 255s on the car. Well, it's the other way around. The 255s on the car were way under-spec. The front 225s on the car were not as under-spec. They were indeed wider than the 215s SPT which is expected.
I couldn't wait to mount the tires and check out the sidewall profile. The front sidewall profile don't look bad with 215 on 7.5 wheel. Looks like it would run even better with an 8" wheel.
The rear sidewall profile don't look as good as the front. The 8.5" is a bit narrow for 245 for my taste. A 9" wheel would've worked better.
For those who think I'm nuts about max-wheel-width, you're wrong. Although narrow-wheel-wide-tire can potentially provide more ultimate traction mid-corner, wide-wheel-narrow-tire provides much linear, predictable and stable handling with lower requirement on -ve camber. With stock suspension (plan to keep this way as much as possible) and stock rear bumpsteer behavior of the S2000, the car will definitely like those plus'es.
Going from 255/40 to 245/40, rolling diameter becomes smaller and the car should look lower but in reality the difference is hard to detect from the pictures, especially when the 255s were worn out and therefore shorter.
However, I used some rough estimate on the rolling diameter change by taking note of the driving distance from work back home, from exact same parking spot, same route, same lane-use, and doing a few times for each set of tires. Surprisingly, I found out that the 255s logged 13.4 km while the 245s logged 13.8 km. I attributed the bigger than expected difference to measuring error.
Can't wait to try them out on the track...
