My number I used in autocross. I drove an AE86 and wanted to use 86 but my buddy was using 86 at the time. Since my AE86 was an '85 model I decided to use my cars model year.
Same here, when I played Forza every car I raced I put 13 on the side of it. Sucks that you can't do it in GT5 but then again GT has always been more about racing, and that's where it's the best.