I really want to buy these, but really no game system to use them for. I know theyd work as chaos cultists but want a mad max system
There are a few rules systems around, besides the classic Car Wars. Off the top of my head: Axles and Alloys 2 (free floating around the web), Road Rage (Stan Johansen Miniatures), Machinas (Two Hour Wargames), and probably a few more. Machinas is meant for races or chases, the others are more free form.
I'm planning on using Axles and Alloys 2, with some house rules I found around and some I've made. Most of these rules are scaled for Hot Wheels/Matchbox sized games, but it's easy enough to convert to 6mm.
Edit - I'll also use them for traditional sci-fi battles as one or two of my factions in a PA setting, along with some of the supertanks and regular PA stuff.