19s or 20s?

[kn7671]

Most people don't realize that the Mustang comes with several different diameter tires, spreading over 1.1" in difference.

V6 with 17 = 28.0"
GT with 18" = 27.2"
GT with 19" A/S = 27.7"
GT with 19" Brembo = 27.2"
Boss 302 rear = 26.9"
GT500 with 20" rears = 27.9"

In this regard, if you had a normal 245/45R19 (27.7") All-Season equipped GT, which is .8" taller than the 285/35/19 (26.9") rears on a Boss 302, and then jumped to to a 20" 285/35R20 rear tire (27.9") would give you .4" shorter sidewall than the stock 19" AS tires, but a 100% equal sidewall height to a Boss 302 rear tire.

Additionally, the 20" tire at 27.9" rear tire will give you a better ride than the 26.9" rears, while giving a little more overall grip from a slighty larger contact patch.