whats the typical rpm of a post hole digger? I made one years ago out of the street cleaner brush motor direct drive, rpm's maybe 120. ths was a small motor but had plenty of torque, used a 9 inch auger. will hook up the bigger motor and check the rpm. since this was the drive wheel (tire mounted directly to the motor) of a three wheel street cleaner it may allready have bearings.
If it carries the weight of the machine it will have bearings of some sort.
I try to get t checking the rpm on my bobcat 15 c auger, I have machines with 15 to 18 gpm
But I would guess 50 to 60 rpm at 17 gpm flow. I remember it was supposed to generate @ 1500 ft lbs torque at 2500 psi.
The wheel motor you have may be made to work at such low rpm with high torgue, however most motors need to run much faster then that.
If you know the machines max speed, you can figure the motor rpm from the wheel diameter, then you should be able to calculate the flow (gpm) your motor needs to run that rpm. Then system max pressure at your given flow will allow you to figure torque.
A 9” bit wold not need 1500 ft to work, maybey 400 to 500, but the bigger the bit the more you need.