Yes, in this particular case it is 50-50 you can stay with a 55 wall or go down to a 50 either one is acceptable but the 55 is closer but still 1.5% off. If vehicle calls for 215/55-18 and you want to put 225s you will need a 225/50-18 or 225/55-18.
If a 235/50-18 would fit without rubbing it would be your best choice and I believe would look mean. This is what I plan to try and fit on mine next summer.