Rootes also had other ratios available to, at least 5.22:1.
Starting from 4.22:1, possible ratios are;
4.44:1, 4.55:1, 4.78:1, 4.86:1, and 5.22:1.
There may be more but these ARE listed in my Hillman service manual. BTW, they are all spiral bevel gear sets EXCEPT 4.22:1 and 4.44:1 which are hypoid gear sets.
I would agree that the tach is probably out of calibration due to the known tendency for electronic component degeneration. There is a possibility the rear gears could be the real problem.
I think I would check the tach reading against a garage's inspection or tune up analyzer machine before getting more involved mechanically.
Just a few thoughts,
did you have only one wheel off the ground ?
4.44:1
???
bob