Game on-Auto Assault.

Wiki:

"Auto Assault was a massively multiplayer online game (or MMOG), developed by NetDevil and published by NCsoft. It combined vehicular combat with role-playing elements, allowing the player to explore a post-apocalyptic future in customizable cars, motorcycles, semis, and tanks. It took inspiration, in part, from the Mad Max series of films.

Players could choose to play as one of three fictional factions — Humans, Mutants, and Biomeks — as well as a class to determine the type of character they would play. The majority of the gameplay took place in a vehicle, but the player could leave the vehicle when entering towns in order to purchase items, talk to contacts, etc.

The game servers have been shut down as of August 31, 2007, and current players will no longer be billed. NetDevil issued a statement shortly after the shutdown news, citing an agreement with NCSoft to buyout the IP rights was not reached

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In 2030, alien craft drop strange terraforming pods onto Earth's surface that release killer contaminations across the world. The contaminations, if not fatal, cause mutations in some humans on Earth.

The resulting mutants are considered a plague and are herded into internment camps, however some mutants escape and form plans for revenge, causing open war. To fight this, the world's governments unite to battle them. These governments create biomechanical soldiers -- immune to mutation -- known as Biomeks that get mental and physical enhancements.
Facing defeat, the humans nuke the world as part of a total purge program. Elite humans are saved, but the rest of humanity is apparently destroyed. Even the biomeks were left on the surface to be obliterated by the cleansing.

The humans live underground for a period of nearly eight hundred years, continuing to advance their harnessed alien technology, developing powerful energy shielding, among other advancements. All the while, they never know that the cleansing almost totally failed. On the surface, small numbers of mutants, biomeks, as well as groups of scavengers and gangs, and even mutated forms of plant and animal life; grew and thrived on the remains of the old world.
Because of this failure, the humans who assumed the world would be clean instead still found Biomeks and mutants on the surface. Open war occurs, but it is pacified over time.

Thus, conflict between the three groups is reduced to small scale battles involving armored cars, trucks, motorcycles, semis and even tanks, armed to the teeth with advanced weaponry. This is the world the player enters and how the game starts."