What is the proper term to indicate how much force the driver needs to exert to steer the wheels of a car?
The force required for the driver to turn the steering wheel would be a torque measured in foot-pounds (or Newton-meters).
Extra Credit: The actual force required for the rack in a rack and pinion style steering system to push/pull the tie-rods to turn the wheels would be a simple load measured in pounds or kilograms. The actual load the tires experience when trying to turn would again be a torque.
I've heard the term "steering effort" used, seems appropriate. It's not quantifiable, but it gets the idea across.