For one quilt I got tired of making the same 12 inch leaf block, so I made half of the number I needed. I cut 12 inch squares to put between the leaf blocks. Since I didn't have enough of any one fabric and couldn't get more, I purchased a coordinating fabric for the squares.
Another option would be to add coordinating fabric to use for sashing and borders then use your focus fabric (the racing cars) for the cornerstones.