What you have to do is go into the .truck file and change the tires in the flex bodies section, add the first line of the material for the tires in the managed materials section and paste everything Dough's BKT file has into the entire truck file.
If it seems confusing watch this video from carkiller 458. He shows multiple ways but if you go to about the 18:00 mark it will show you how do it the way I found it the easiest.