Have you tried "Case 5" in the AF menu?
I am not a motor sports photographer, though I do try it occasionally, but I find this mode works well for birds in flight. Actually I use it for everything, even landscapes!~ Yup that is servo AF (case 5) for landscape, it works and I am too lazy to faff about changing things, in focus is in focus!
I was down at the Pembery circuit about a month ago and banged off 2800+ images - sorry I got a little excited. Except for when I was mucking about with VERY low shutter speeds My 1DX missed focus on 2 frames - thinking of sending it for repair! The majority of my shots were approaching and crossing/panning shots, but a number were of cars going away with lots of dust in the way - no issues. For reference my lens was a Canon 300 F2.8 L IS Mk1, and I only use BBF.
Give Case 5 a try, I didn't like case 3, I hope it works as well for you as it does for me.
Life is for living, cameras are to capture it (one day I will learn how!).