Why does a speedometer measure instantaneous speed rather than instantaneous velocity?

The speedometer doesn't indicate anything about what direction the car is going. It doesn't even register negative values when the car is moving in reverse. Therefore its name speedometer is apt. It measures speed (the magnitude of velocity), but indicates nothing about direction of travel.

Good organization of the material, simple answer are on this site: http://www.physicsclassroom.com/class/1DKin/Lesson-1/Speed-and-Velocity