If you decorate any cake to resemble Jeff Gordon's car you will be violating copyright. Most of the Wilton cake pans say "for home use only" on them, too. Although I don't see how you can copyright a "car" shape...unless it's a specific car, ie a corvette.
If you decorate it using the number "24" and his colors, it will probably be copyright infringement. I would definitely write him and ask. These guys are generally pretty good to their fans so he will probably say "okay".
Leslie
NASCAR Dale Jr. Fan
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