Nicely done... Panning with a subject like this is often difficult because of the vertical motion imparted by the vehicle bouncing over the rough terrain as well as trying to get the background blurred while keeping the moving vehicle as sharp as possible.
Often arriving at the exact shutter speed which will accomplish this is a matter of "guessing and hoping". In reality, it would probably be more of an artistic decision rather than a scientific choice...
I wonder if there is any camera setting which would result in a burst of shots in which the shutter speed would vary but in which the aperture would also vary so that the exposure would remain constant.
If a photographer used aperture priority and exposure bracketing, the shutter speed would vary but, so would the overall exposure. However, if you did use a 3-shot sequence in aperture priority, the shutter speed would be right on, 1/2, and 2x. I am thinking that most cameras have a wide enough exposure latitude that the one-stop over or one-stop under would not ruin the image. The next time that I am going to do a panning sequence, I may try bracketing
What do you think?