Does anybody know what the largest width tire I could fit on an 18 x 8 rim is? I don't want to know if it will fit on the car, just if it will fit on the rim.
Depends what the aspect ratio of the tire will be.
For instance, if you will be using a 40-series tire, the generally accrepted max tire width is 245mm. 50-series tire you can likely move up to a 255mm tire.