* Velocity is a vector: Velocity includes both speed and direction.

* Turning changes direction: When a car rounds a turn, its direction is constantly changing.

Therefore, a car cannot have a constant velocity while rounding a turn. It can have a constant speed (90 mph), but its velocity will be changing due to the changing direction.

To be more precise, we could say:

* The car rounds the turn at a constant speed of 90 mph.

* The car's velocity is changing as it rounds the turn.