Mopar Action had an article about this some time back.
R.E.s recommendation was also to check the odometer first.
By checking it against mile markers as Rapid suggests.
Normally if speed is some percentage off, number of pinion teeth will be off a like percentage.
On your car I would have started with 27 teeth, which normally would have you pretty close.
So I'd think yours would be reading low, not high!
Sounds like your speedo is off.
There used to be shops that could calibrate mechanical speedos, not sure if they exist today.
If you could borrow a speedo from someone that would be my next move.
Let us know.