17" are really the way to go ... If you start getting into the 18's (and we are talking about looks here, and this would be my My Opinion) than yes, they fill the 'outside' of the wheel wells up, nicely. However, the way that most Mustang wheels are designed they have alot of open space within the wheel itself, and opting for the 18's it leaves it more open Inside the wheel. IMO, go with the 17's and get the cheaper tires, etc... and use the larger sidewalls of the tires to fill the 'outside' portion of the wheel wells, and the 17's to fill in the 'inside' (or center) portion. ... I went with 17's on my car; very satisfied.