High RPMS at 55 MPH

Steve we already posted this.........first thing to do is determine the rear gear, and you can READ the tire size

Jack up one wheel if you don't know if it's limited slip. With car in neutral and parking brake off, can you turn one wheel? Yes? Turn the wheel exactly TWO turns while counting the driveshaft. That is the rear gear, IE 4 turns and "a little" is a 410, etc

If you cannot turn one wheel, IE sure grip, jack up both rear tires. Turn either tire ONE full turn while counting driveshaft turns. Same deal, 4 turns, etc is 4.10 and so on

Trans is 1:1 in 3rd make SURE it is shifting into 3rd. Torqueflites are "easy" there are no hidden OD gears, etc. It's either in 1, 2 or 3rd, and or slipping.

With tire size and rear gear, consult an internet www calculator.

SUSPECT the tach. You have a GPS? Easy Use it for a speedo to compare