If you believe that the tire flying off was caused by something the mechanic did wrong, or something he failed to do (e.g. that he did not secure the tire properly after doing the work), you can sue him for the damages (property damage--e.g. cost to repair, or the value of the truck if totalled; any injury; cost to tow; lost wages) you suffered as a result. For several thousand dollars of damages, as it sounds like you probably suffered, if you do not have insurance picking up the bill (e.g. collision coverage), you should retain an attorney for the lawsuit; if it's only several hudred dollars, you might represent yourself, such as in small claims court.