The reason I asked is because my car's lowered with hotchkis springs, and I'm getting a 1st gen veilside kit. I'm afraid I might scrap the ground with anything smaller than 19"
it's just a matter of tires, then.
stock 16s with stock 205/50/16 tires are ONLY an inch smaller than 18s with 205/40/18 tires.
here, this tire calculator should help:http://www.miata.net/garage/tirecalc.html