If the wheels are the same size it doesn't mean you won't loose an accelleration. The 18's i chose, light wieght ssr gt1s lowered my 1/4 time from 13.7 to 14.0-14.1. BUT, its worth it!!!! I'll never go back!

Brandon

 

Actually Brandon you raised your 1/4-mile time. gotcha!
Seriously though, several points. Your butt meter, or anyone else's for that matter, isn't calibrated to sense minute tenths of a second which is usually the amount of time affected by tire/wheel decisions. We're not talking adding a supercharger here so yes, in all but the rarest of cases, you will DEFINITELY create a slower accelerating S2000 with your choice of 18's. Bear in mind your tire weight also will change though if using a 35 series, you can usually match OEM tire weight if using a 235mm or smaller tire. You can help offset added wheel weight by using tires noted for their light construction. Toyo Proxes T1-S (my choice) is usually two pounds lighter per like-sized tire than the competition. Unfortunately, there are few areas online which gather tire weight data so you're on your own there. In my own case, my 18-inch 225/40/18 tire/wheel (Velox) combo weighs 194-lbs total. My newbie combo (sitting in the spare bedroom waiting for spring) weighs 132-lbs total which breaks down to 33-lbs on each corner and yes, that's the tire AND the wheel combined! We're talking nearly 60-lbs (I have 17's on the front) of unsprung weight which is akin to 150-lbs of static weight (give or take a percentage point or two). So, the day I exchange tire/wheel combos it will "feel" as if I have dropped 150-lbs of weight from the car and with an amount like that, I expect to feel the difference immediately. Such a weight drop in a car weighted in the 2700-2800-lb range should create a performance increase equivilent to 12-15hp. My tires weigh 19.4-lbs and the wheels weigh 13.6-lbs compared to 25.2-lbs for the tires on my 18's and 25-lbs for the wheels.
In the end, you'll probably lose a few ponies going with your 18's but the usual benefits (asthetics, quicker response, etc) will probably make you forget about the missing horsepower, however slight it might be.
One thing I've learned over the past three decades of owning sportscars is losing weight is "free" horsepower. This explains why the Lotus Elise is such a great performer. Another example would be the new Vauxhall Turbo, whose performance is in "supercar" leagues for quickness off the line. On my MR2-Spyder, I managed to drop my 0-60 times down to consistent mid 6's just from weight losses alone. Hell, can you imagine a 240hp, 2500-lb S2000?
You pay the price for 18's in most cases, unless of course you want to pay the big $$ for Japanese lightweight wheels. I think this is why so many choose 17's. One more thing. A 17-inch wheel whose spokes extend to the lip will have the appearance of a larger wheel whose spokes end well inside the lip. A visual sleight of eye so to speak.

 

18's are slower for sure. I have pretty small tires and the OD is still bigger than stock. The wheels are lighter but the overall wheel/tire combo is not that different. Still, my 17" Volks made my car feel FASTER than stock (lighter, same or slightly smaller OD). My new 18" Volks definitely make the car slower.

 

Which is commonly called the polar moment of inertia for the wheel. Even if the wheel is the same weight (which 18s in most cases will not be), they can be distributed better (or worse) than what you have.

I went with SSR GT1s in 18s and definitely noticed a little bit more drag. But they look pretty slick.

 

Yea... ssr's are pretty slick..... gt1's anyhow