The average speed is the ratio of distance covered to the time it takes, or the average of the final and intial speeds. The change in speed is the difference between final and inital speeds. The symbol v is used for speed because speed is the size of the velocity vector . Time is one of the undefined qualities of physics. We all know what it is and agree to use a second as one unit of time.
Voltage is an energy measure, the energy carried by one coulomb of electrical charge.
The voltage between two points in a circuit is the amount of energy available for pushing each coulomb of charge from one of these points to the other. Work is done on an object whenever it moves because it is pushed or pulled. Work increases the energy of the object.