High RPMS at 55 MPH

Ok, finally got a chance to chk things out. Hooked up another tach so I could read it and the factory one in the car at the same time.
Tire size 245/60/15 = 26.5" tire diameter
Rear end gears 3:23= jacked up car turned tire 1 revolution=3.25 approx turns on driveshaft
I "flashed" the stall, and it looks like approx 1800 on new aftermarket tach.
MPH/RPM readings
Idle rpm 1000 factory
500 aftermarket
30 MPH 2300 factory
1600 aftermarket
40 MPH 3100 factory
2000 aftermarket
50 MPH 3700 factory
2500 aftermarket

So to me everything looks good?? Other than dad paying to have the factory tach redone with upgraded electronic board and recalibrated and it looks to be way off.