18's are purely for looks. 17's are for performance/looks. Toyo and Falken have been the tyres I've seen on 18's for our cars. Toyo's being my preference. I'm going 215/40/R17 Yokohama A.drives when I have the 17's on. For me I'm putting the wheels on before I consider lowering. The wheels are the main measure point, so without them on before you lower you won't be able to tell how it will sit. I am running coilovers though, so it's more important for me to put the wheels on first then lower due to the fact that i can adjust my ride height and pin point it how i want it.