How does the speed of the combined cars compare to the speed of the car before the collision when a parked car is hit by a moving car and the two cars stick together?

The combined VELOCITY of two cars that crash will be somewhere between that of the individual cars. In this case, the combined speed will be less than the speed of the car that was moving before the crash.If you know the velocities and the masses, the exact speeds can be calculated using conservation of momentum.