Final answer:
Average speed is calculated by dividing the total distance by the time of the trip, whereas the magnitude of the average velocity also considers the direction of travel. These quantities are the same when traveling in a straight line without changing direction. The car's instantaneous velocity matches the average velocity when its velocity is constant.
Step-by-step explanation:
When you divide the total distance traveled on a car trip (as determined by the odometer) by the time for the trip, you are calculating the average speed. However, if the displacement from the starting point is also considered, then dividing that displacement by the total trip time gives you the magnitude of the average velocity.
Average speed and average velocity are different in that speed does not take into account the direction of the movement, whereas velocity does. The average speed is the total distance divided by the time elapsed, regardless of the path taken. In contrast, the average velocity considers both the magnitude and the direction of displacement over the elapsed time. These two quantities are the same when the travel is in a straight line in a constant direction because the distance and displacement are equal.
As for the initial question, the instantaneous velocity of the car would be the same as the average velocity for the entire trip in a situation where the car has been traveling at a constant velocity. That is because there are no fluctuations over time in the speed or direction, hence the average and the instantaneous velocities will match at any given moment during that trip.