The speed of your hand panning needs to match the car's speed. Not faster, or slower as in your case.
Of course, this takes practice, practice, and more practising. If you want the whole car to be in focus, the best way would be to keep it centred in the viewfinder.
Oh and do you think the neon lights are a little attention-grabbing? Or do you like it that way?
1/15 - 1/30 would be ideal, and I personally prefer 1/20 but it really depends on the vehicle's speed.
f8 would be a good compromise between a sharp car and a blurrer background but increase or decrease the f-number if more or less or the car fills the frame.