I have seen a lot of examples of distance measurement but none where they found the speed of a moving object
Speed is just rate of change of distance. It's simple calculation.
Your problem may come from the rate that you can take measurements.
Speed guns work continuously, calculating speed from the Doppler frequency of the echo.
However ultrasonic rangefinders make discrete round-trip time measurements, limited by the speed of sound and the sensitivity of the receiver - you may only manage 15 to 20 measurements a second.
At scale ranges, this may equate to very few measurements.