Your speedo isn't too fast your car is too slow.
J/K
This is what I do since since I never have the chart handy or I have freekish tire sizes that never fit the chart.
Get on the Interstate and right down the Odometer at a mile marker sign. Drive a long distance, 50 to 100 miles uzing the mile marker signs as the distance. At the 100 mile marker right down the odometer and calculate the distance as a percentage. i.e. 100 miles actual/110 miles Ododmeter = 10% difference.
I then get a new gear with 10% less teeth if the speedometer is reading too slow or 10% more if it is reading too fast.
Works for me.
A great time to get that mileage is to come to the Mopar Show this weekend in Nashville.