That is a "TRX" wheel from the handling package (as unit mentioned) and they come out to be approximately 15.3 inches in diameter and like 6" width. The ONLY replacement tires available are $200 michelins. Don't waste your money, a better investment would be some 16" pony wheels, later style "turbine" wheels or even Cobra R style wheels.
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'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT