SHELBY GT 350 Member
Joined: Jan 2011
Location: Central Massachusetts
I think you should be OK . . . but you need to compare the new tires that you are considering to your old/existing tires, not to some other different tires.
I'm pretty sure when you check your old/current 17" tires you will find that they are about 27.2" diameter (that is the standard factory diameter for pretty much all of the S197 cars, as far as I know)
So the new tires are about 1.1" larger diameter (also called "taller") than your current/original tires, or 1/2" on each side (radius). That is not a big deal and should clear no problem, assuming the wheels are sized correctly, which mainly means the correct offset so they are not sticking out of the fenders farther than they should. If they are Ford factory Mustang GT wheels, that should not be an issue. Believe it or not, I'm pretty sure Ford still sizes the tires and wheel wells so that you can use tire chains and still have plenty of clearance! So as long as you won't be using chains, you should be fine.
You can do the speedometer math as follows: 28.3/27.2 = 1.04, or 4% increase . . . this means the car will travel 4% farther for each rotation of the tires, and the speedometer will read 4% low (assuming it was correct before).
| 2010 GT coupe | Daily Driver with some Steeda and GT500 take-off stuff for the occasional track day