I have a 93 LX with 14" wheels and plan on putting on a set of 16" Pony wheels. From what I just read, the stock tire size on the GT or 5.0 using the 16" wheels were 225 55 16. However, I also read, the inner plastic fender skirt was reshaped on the GT and 5.0 to allow clearance for the larger wheels and tires. Now knowing this, it sounds like if I change the 14" wheels to the 16" wheels and use the standard tire size of 225 55 16, the tires will hit the wheel skirt since my LX does not have the reshaped wheel shirt since it came with 14" wheels. My question is, can I use a smaller tire on the 16" wheels to avoid an issue with the tires hitting the fender skirt?
You can go with a shorter tire, but you should be fine with what you have. The 91 to 93 mustangs had a little bit bigger of an opening for the wheel well to take care of the rubbing issues with the older models. Mine is a 90, and I'm running 245/45-16s on the front of my car. I did have 245/50-16s but when they wore out I was able to find something shorter and now the only time it rubs if I have the wheel cranked all the way, left or right, like if parking, but it rubs on the a arm instead. I try not to turn that sharp anyhow.
You should be able to search the tire diameter for the tire you have, or google a tire size calculator and find out what it is.