I would not suggest adding larger diameter wheels on the ST. I have noticed the Fiesta is highly sensitive to weight changes with wheels. Any extra rotational mass needs to be accelerated and stopped. I also think it will be hard to use 18" wheels on the street with the ST. I run 17" wheels with 215/40 17 tires and that is on the edge of what will work for a diameter. Things get a little close on full bump with that diameter.