A car only needs a fan while stopped and below 25 mph. If it over heats at speed i would look at a couple of other things. It could be a plugged radiator or semi plugged t-stat. Water pump could be bad as well.
Another thing that gets over looked is the age of the hoses. i have seen the bottom hose go soft and close due to the water pump sucking it closed. A quick fix is to go to an auto parts store and get a spring that comes in some lower hoses and insert it into the bottom hose.
Most important: never mix an electrical fan with a mechanical fan. It will sometimes cause a rsonance which makes the air go back and forth across the radiator which doesnt allow the air to do its job at cooling.
