There are several places you need to be concerned about in the fitting of tires/wheels.
With a 15" rim, I cannot say for sure (never done 15"). But several epople I know run 245-50-16 on the stock Pony wheels.
I ran 245-50 and 245-45's on 16x8" wheels,
without really worrying.
Finally I upgraded to 275-40's on my 17x9-1/2" wheels. Now THESE needed some masaging of the fenders...
------------------
Darius Rudis
Home built t-arm/panhard suspension powered by an S-trimmed motor :-)
My Mustang Page