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If the speedometer of your car reads a constant speed of 40km/hr can you say 100% for sure that the car has a constant velocity? explain your answer.  

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Velocity is speed in a given direction. Because it requires that direction be included it's a vector quantity. Speed is distance covered divided by time elapsed, and isn't a vector quantity.

The first factor to consider is if the car is maintaining a speed of 40 km/hr. For any time interval that this speed is maintained, the speed is constant. 

The second factor to consider is if there's a change in direction. If the car maintains a constant speed but changes direction during that time interval then velocity is not constant. Acceleration, which is change in velocity, can be a result of a change in speed over a time interval, a change in direction over a time interval, or both. A car going in a circle at constant speed is accelerting. 

You can only say for certain that the velocity of the car is constant if the speed and direction are both constant. 

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