The top speed of the fastest 289 option Mustang (the K-code, 271 hp) was about 115 mph. Yours must’ve been quite modified. Even the highly modified ‘65 Shelby Mustang GT-350 topped out at only 135 mph.
Nonsense!
The ‘65 coup was a 271 HP that easily hit 140.
Had a 350 too later, that exceeded the limit of its speedometer at around 172 MPH.
It mostly depends on the rear gears, not the horses. Even the 225 HP V8 would exceed 115 MPH with .323 gearing.
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The Hertz version of the 350 was electronically rev-limited to keep the top speed down, since it had higher rear gears (.292) to go with the automatic, otherwise it would deliver some exhilarating top speeds.