No body object or point can have a constant velocity if there is a change in velocity aka acceleration.
We know that the derivative of velocity is equal to acceleration.
This means that if you take the derivative of any constant velocity : 2mph, 5mph, 85mph, etc, you will always get the value zero for acceleration. This includes the constant zero as well. In this example we would take the derivative of zero which is zero.
Comprendes?