Stock on the 18's are 275/65/18, should measure a hair over 32 inches tall (32.1). A 275mm wide tire is roughly 11 inches wide, if I were a betting man and those are 33's on that truck as well as 18's chances are they are a 33x12.5x18.
EDIT: Had to correct myself, no way a 33x12.5 is going to mount on the stock wheels. That would require a 9 inch wheel min. Can I change my bet to either a 275/65/18 or a 285/60/18
