Your speedometer may be off. The faster you go, the more it's off in mph. Do you have different gears? A non-standard tire size?
If the fine is based on how fast you were going and there is a difference between 10 over and 16 over, just tell the judge you are willing to plead guilty to going 75mph because that is how fast you were going.
If the fine is the same no matter how far over you were going, just pay the ticket.
It does suck that they would pick on people in Mustangs. Here in Minnesota, it only seems like the pick on ricers.
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351W 89 Mustang GT Convertible