Those fan-on settings are too cold for any thermostat, even the 160°F thermostat. Once a thermostat opens it should not open and close chronically; it should open wide and stay wide open til the powertrain is done working.160°F thermostats are fully closed below 160°F, and fully open by about 176°F.
If you want the fans CONSTANTLY on for some strange reason? Fan-OFF @ 160°F or less.
With a 160°F / 71°C thermostat:
Set the Fan-OFF @ 176°F / 80°C. Any lower and the thermostat will stutter.
Set the Fan-ON @ 185°F / 85°C. This might let the fans shut off while the vehicle is moving fast enough to do the fan's work for them (45MpH or more).
Be sure the radiator (and external ATF cooler if also equipped?) are getting as much airflow into them as possible so that the fans' work is made easier.
Check your baffling, and consider adding coroplast in choice spots if need be.
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