Yup... using a calculator is the smart way, otherwise you have no idea just how tall the tire is.
btw, 225/40-18 is about .7" taller than a 235/40-17 (or .63" taller than stock). Those are wagon wheels... nearing the height of Cadilac tires! I find it hard to believe that that sized 18 would handle better than the 17" size I listed. Unless there was crappy tires on the 17" and decent ones on the 18", but how much weight are you gaining with a 1" taller rim and .7" taller tire? For 18's, I'd think 235/35-18 would give you better performance, but with that sidewall you might as well just pour rubber cement on the rim and forget the tire.
