Not quite sure what you mean there, got any example pictures?
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For the three car design, I think the whole graphic will have to be scaled down to fit on the shirt. We had that problem in the last couple of contests and some designs had to be scaled. You guys doing the art might want to change the sizing of your designs to save yourself some work down the road and better reflect the constraints of the final product.
If three cars is too much you could always add the electric mini or one of these things:
https://pic.armedcats.net/b/bl/blind_io/2010/05/24/02g1.jpg
Or the Volt - although I think the golf cart is more consistent with the Leaf and Prius. If you add one more you can make it a 2x2 matrix instead of 3 across.
Another options would be to just to the Prius in profile (very distinctive) and a Lamborghini - so it's just two iconic cars and very simple.
A third option would be to do the three shit cars on the front and a supercar (or three) on the back
These ideas free to anyone who wants 'em.