Nuclear Renaissance. You can get the basic rules for free:
http://www.ramshacklegames.co.uk/nuclear/downloads.htmlTHe basic rules work really well for getting to know the games. Its designed for group play, so 2 to 6 people works fine.
You build a gang by taking a basic stat line and adding abilities, weapons and equipment.
Vehicles are handles in the same way, and really are integral to the game.
I did an expansion which adds loads more gang types, loads of upgrades and most importantly a campaign system.
THe game give syou LOADS of options, so its easy to make rules for anything you want. Zombies, robots, powerknights, tanks, biker gangs, monstaer/beast gangs, mad max style stuff, anything really....