The units for distance and time can vary. Average speed is the total distance travelled divided by the total time taken for the trip. For example a 100 mile road trip might take 2 hours. Remember that instantaneous speed is simply the speedometer reading at any given moment.
Constant velocity means that neither speed nor direction can change.
A car can move with a constant speed and yet change direction. Recall the example of the car turning a corner in front of a radar detector. Only when direction does not change. Because of air resistance, objects dropped in air can only approximate free fall for short time periods. Deceleration to the north is the same as acceleration to the south.