Autobot leader Optimus Prime’s alternate mode in the 2007 Michael Bay’s blockbuster “Transformers” is a Peterbilt 379 Extended Hood truck. Three modified Peterbilt 379 tractors appeared in the 2007 sci-fi adventure film “Transformers” as Autobot-leader Optimus Prime’s vehicle disguise mode.
There was much controversy over this in the Transformers fan community because Optimus Prime is almost always portrayed in the various cartoon series as a flat-nose cabover semi-trailer truck or later as a fire engine.
His movie incarnation would be the second with an extended nose conventional cab following his role in 2002-03’s Transformers: Armada. The filmmakers have said that, ignoring the fan-termed “mass shifting” associated with the series, Optimus’s final official height of 28ft (almost 10m) in robot mode would not have been possible to allow for all of the mechanics of the CGI model if they had gone with a cab-over, which would have dropped him instead to a smaller 20ft (almost 7m).