The Transformers was a blockbuster hit in 2007. The movie had an agreement with General Motors and used GM cars for the Autobots. The exception was Optimus Prime and Barricade (Optimus Prime is a tractor-trailer). Producer Michael Bay went through GM's concept car vault and found the Camaro for Bumblebee and other forms to complete the Autobot team using GM vehicles. The cars included were an H2 Hummer, Pontiac Solstice, GMC Top Kick and the Peterbilt 379. The movie inspired Chevrolet to release the alternate form of Bumblebee as the new Camaro.
Optimus Prime originally was a cab-over tractor-trailer. The truck turned to the robot and the trailer turned to a command station. The 2007 movie remade Optimus Prime into a 1994 Peterbilt 379. The Peterbilt was originally just a road truck untill the production team repainted it and added a lot of extra chrome.
Ironhide in the original series was a red Toyota Van that separated during transformation.The front half of the van made into his robot form and the back half became a mobile cannon. The movie remade Ironhide into a 2006 GMC Top Kick, a GMC heavy-duty truck.
Ratchet was originally a Toyota van painted to look like an ambulance. The movie made Ratchet into an H2 Hummer ambulance. Ratchet, being the Autobot Medic, needed a medical form for his transformation. The production crew tore the Hummer down to the frame and rebuilt it with an ambulance body.
Jazz originally took the form of a Porsche. The movie remade Jazz into a 2006 Pontiac Solstice. They modified Jazz by putting a rear spoiler on him.
Decepticon Barricade was originally supposed to be Autobot Prowl in the movie. The production team decided to use a 2006 Ford Mustang Saleen for Barricade. They painted the Mustang to resemble a police car.
Bumblebee was always remembered to be a yellow Volkswagen Beetle. The production team wanted to make Bumblebee more of a warrior and went with the Chevrolet Camaro.