While sorting out cooling issues that came with my 82 diesel conversion, I
found out that it had a 2-speed fan but only a 1-speed fan switch.
Replacing the switch on the radiator with a 2-speed took care of that
oversight(plus rewiring a bit...someone tried solving the overheating
problem by wiring the fan to be on 100% of the time the ignition was on).
If you have three wires coming out of the fan, you have a 2-speed fan; 2
wires means a 1-speed fan. Same for the sensor/switch on the radiator: 3
wires = 2-speed switch, 2 wires = 1-speed switch.  You could test by
warming up the engine first, turning it off, then applying a heat gun to
that portion of the radiator to see if you can trigger to two fan speeds.
If you have an infrared thermometer, you can also read the two trigger
temperatures easily; $12 at Harbor Freight...a dandy little tool to have.
I wouldn't expect the fan to make much difference in cooling at highway
speeds unless you have a significant tailwind.

--
Brett in Portland, OR
"Albert" '82 VanaFox I4 Riviera