I found 215/45 17 to be very close to hitting the bumper cover and inner fenders. It also made the Fiesta look really tall. I run a P215/40 17 on my 2011 Fiesta and had them on the ST for a full week and had no issues. For winter, go with the smallest and narrowist wheel possible. It reduces your contact patch and provides great wheel protection. In the summer, wider is better and in the winter, narrow is best. You will also get a ton of snow/ice build up with a 17" wheel since it won't take much accumilation to exceed the aspect ratio height. I used to have horrible issues with this when running a 16" snow tire. With the 14" I have on my 2011, it almost is never an issue. Pick a wheel with the smallest opening between the spokes as well.