After removing the numbers you can remove the remining glue residue with a quick wipe of acetone. Works like a charm. If you are putting new numbers on, go to a print shop/sign shop and they can do your new numbers on a vinyl transfer in a few styles and colors. With all the numbers and letters on one sheet, the spacing is perfect. You can also pick a letter styling, but dont get so fancy as to prevent the authorites to see them easily. I usually put mine below the rub rail.
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