You can give the tires a light sanding. Depending upon how old and used you want 'em to look, use 400 to 600 grit on the treads and 1200 to 2000 on the sidewalls, or you can spray the sidewalls with dullcoat and rough up the treads a bit with the sandpaper of your choice (start with 600 and use progressively rougher grit as desired). If you want to retain the raised lettering, you probably don't want to sand much on the sidewalls. Try giving them a good rubbing with a clean, cotten cloth to dull them out a bit before spraying or sanding.
Glenn