i am geting ready to buy new rims but i was wondering if you could run 18's on a tgp without rubing?
Yes, as long as the offset is right, and the tire size is fitting.
http://www.miata.net/garage/tirecalc.html
Go there and enter the stock tire size and then enter the tire size you want for 18's. I run 245-40-18's on my Arelli's and it will only rub the plastic in the wheelwell if I have it turned to lock and go over a bump. If the car is lowered, I would then start looking at lesser sidewalls.