Looking to get some new wheels and rubber for 03 s2000, the question i have if i go up to 17" rim with rubber will this throw off my speedo? I know the o.d. of the tire is bigger so that means the speed will read slower? am I right on this?

 

one inch wont make a difference, maybe if you put 22's on lol

 

OD of the tire won't change if you go with the right tire sizes. Either 245/40-17 or 255/40-17 (AP2 and CR sizes).

If the tire OD difference WAS 1", that would give you a 4% speedo/odo error and 4% "worse" gearing, but that's not the case...

 

ZDAN is correct. Just get the correct size tires and you will not have any error. 245/40R17 and 225/50R16 are basically the same diameter.
If I can help with the tires let me know.