The stock wheels were probably 225/70/16 which gives you a diameter of 28.4". You want to stay as close to this as possible in order for your speedometer to be correct.
Unfortunately there isn't a common tire size for an 18x7.5 or 18x8 rim that hits a 28.4" diameter. Your best bet is to go with either of the below options:
225/60/18 = 28.6" diameter
235/55/18 = 28.2" diameter