4x4 tire tread is hard to do no matter who you are...in my opinion anyway.
George Trosley has a great how to do 4x4 in his how to toon cars book.
http://www.georgetrosley.com
On thing I've found that helped me in the past is to exaggerate the recession. On the far side (side farthest away from the eye) pinch those points and drop the tread shape back. remember it curves so curve it way back.
Very nice piece. Nice perspective. Nice style.
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-lemorris