Buying Tires with new rims. What exactly does the " 245/40 - 18 "mean? 99' Mustang ?

245 = the width of the tire tread in milimeters. 40=the aspect ratio meaning the sidewall height is 40% of the tread width. You are looking at a wide low profile tire. It should improve your handling, be careful of pot holes - the low profile means the rim is closer to the ground. The Anthracite rims look good!

245 is the width of the tire in millimeters, and the 40 is the height of the sidewall: 40% of 245 mm.