I think my stock rim can only fit 225/45/17 max.Yes of course, 225/45/18 or 235/45/18 front on 18x8jj and 255/40/18 rear on 18x9jj is perfect. But you'd need lightweight rims to keep the weight down.
235/40 is of course bigger, coz the sidewall is really taller/thicker:
225/40 = 22.5cm x 40% = 9 cm
235/40= 23.5cm x 40% = 9.4 cm
Profile is just aspect ratio to the width, not an absolute fixed number.
Will use same size so can do tire rotation.