any lightweight 18's for this car?

DR_

First off wheel diameter is irrelevant. Tire diameter is relevant but mostly only to the shape of the contact patch, not much to the total size of the contact patch. Do some research before calling me out or at least link to your sources.
There are some other smaller factors involved like tread design and softness of rubber but for the most part the total size of the contact patch is determined by the weight and PSI.

from wiki
Statically, the size, shape, and pressure distribution are functions of many things, the most important of which are the load on the tire and the inflation pressure:

The larger the load on the tire, the larger the contact patch.
The larger the inflation pressure, the smaller the contact patch.