Ultrasonics do not have long range so you have very narrow window of operation.
either you use Ultrasonic, laser or uWave
There are two methods.
One to measure doppler's shift and second is Equal time interval ranging.
in second method make a good range meter and calculate range for say every 100 ms make average and calculate the speed by v=d/t simple formula but remember in real practical environment this speed is not accurate due to presence of cosine error in your way of measurments