In Hylton's book, Woodworking with the Router, he describes and has plans for a circle cutting jig that will cut a small as a 1" dia. plug up to about 11". I made one in about 2 hours.
But, for the $$, Bob's suggestion would work, too. With Hylton's jig though, you could make the grooves on the face to make a more realistic tire.
'The true soldier fights not because he hates what is in front of him, but because he loves what is behind him.' G. K. Chesterton
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