I have a 00 i30t and replaced the orginial 17" tires with 18". How does one go about knowing what is the optimal tire pressure on different rims/tires?

 

Trial and error. The rims don't really make any difference. It is the tire that makes the difference. How it is made, the materials it is made from are the determining factors along with how soft or harsh a ride you want.

Low profile tires need to have more pressure so that you don't bend your rims when hitting a pothole. You will have to live with a slightly rougher ride because of this.

Start with 35 psi in the fronts and maybe 32 in the rears and see what you think.