Your fan actually came on at those speeds? If my car's moving at more than about 20 mph the fan never comes on.
Car normally runs at 185* w/180* thermostat. Fan turns on if temp reaches 195* and off at 180*. Radiator is a 26 X 19 Griffin aluminum cross flow.
I have a 180* thermostat. You said you have a 160* thermostat. That means the the thermostat is supposed to start opening at 160 +/- 2* and be be fully open at +20* or 180*. If the motor runs at 180* your thermostat is wide open all the time. If your low fan comes on at 175* but you run at 180* the fan must be on all the time? What's the purpose of a 160* thermostat if the car runs 180 anyway?