First, you need to understand what those numbers mean.
Let's take the 245/45ZR19 that come on the 2010-2014 GT Premium.
245 = Tire Width in mm (millimeters)
45 = Aspect Ratio, or a Percentage of the Tire Width to Sidewall Height.
ZR = speed rating
19 = wheel diameter
Now let's dig in on the Aspect Ratio of 45:
245mm x 45% (or 0.45) = 110.25mm <---- sidewall height in millimeters
Let's convert the 110.25mm Tire Height to Inches:
110.25 divided by 25.4 = 4.34" <---- sidewall height in inches
Now let's add the sidewall x2 to the wheel diameter:
4.34" x 2 = 8.68" <<---- Total Sidewall Height
8.68" sidewall + 19" wheel diameter = 27.68" <<<------- TIRE HEIGHT
Go look at the tire specs on www.TireRack.com
for a 245/45ZR19, and you will see that almost all are 27.7" tall, as the manufacturers simply round to the closet 1/10th of an inch.
Now to your question, how tall of a tire can a Mustang accommodate on stock suspension, and secondly, while maintaining handling and performance.
The rear of the Mustang can accomodate a 28.5" tire, but it's dependent on wheel offset and width. If you are using the stock wheels, there will be no problems. The front of the Mustang can handle up to a 28.2" tire before you worry about rubbing under the right conditions, but again, dependent on the wheel being used.
One last thing, every tire made has an approved wheel rim width, and the specs are available on the tire manufacturers website or on www.TireRack.com
. For example, the 245/45ZR19 tire that came on the 2013 Mustang GT Premium can fit wheel widths between 7.5" to 9.0". Ford uses a 8.5" wide wheel with the 245/45ZR19 tires.
Where does that leave us? We need a tire that is a little over 28" that will fit on a 8.5" wide wheel. The only tire that fits the bill is a 255/45ZR19 at 28.2" tall, just 1/2 taller overall than the stock tire. Lastly, this small difference will not affect performance or the speedometer a great deal.