Sounds to me like they are 2.73's. I have 3.08 and run 70 mph at 2000 rpm in 5th. Your 15" tires come out to almost the exact same diameter as my 225/55R16 so I don't think the tires would be enough of a factor drop 10 mph. What is the code on your door tag? (driver side).
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Scott93GT
1993 Mustang GT
Silver CC/Titanium Accent GT, Black Clothe interior.
5 speed, 3.08's, 3 chamber Flows, K&N, Hurst Competition Plus.
1991 F150 4x4 XLT Lariat
Six Stick Regular Cab LWB Tow Vehicle