On a 2004 or older I'd agree with you. I won't go larger than 18 on my Supra, but the 2005 Mustangs were designed with larger overall wheel/tire setups. They look good with 20s where most cars look like they have wagon wheels on them. In fact, the Saleen 2005s are coming with 20s. Now, 20s will limit your choices in tires more than 19s and be significantly more expensive.

This is the pic that got my attention. I would not have believed the wheels on it were 20s had it not been written on the ad.

 

I went with 18" ,,, alot of people think I should lower mine, but I like it the way it is,, If you look at older Mustangs, they also did have the gap , which is part of the retro look I like about the new 05,.,, IMO loweing the car would take away from that look ....