High RPMS at 55 MPH

As others have said do not assume anything

I remember you had troubles with a tach board, was it this car?

1..........First verify the rear gear, and compute what speed should be with the tire size using an online calculator. To do that jack up ONE rear tire, put the trans in neutral, make sure brake is off and see if you can turn the wheel. If you can, Turn the tire TWO revolutions while counting turns on the shaft. Easy. 3.23, with two tire turns, the shaft will turn 3 and about 1/4 turns

3.55, shaft turns 3 and just a shade past 1/2 turn, etc

Then take your gear and tire size and find an online calculator, just google speed, rpm, etc there must be millions

2......Then you must be certain what gear it's in. Auto? should be able to feel it shift twice, and a little playing with the shifter at 35-45 mph should tell you what it's doing.

3.....Use a GPS unit to verify what the speedo reads

4.....Borrow? a tach and see what it's doing. I checked my tach with a sound card lashup and a downloaded signal generator, which produces various musical tones.