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.