Autokill is a set of free wargames rules for vehicular combat.
Steve Jackson Games has made the rules for the Classic Car Wars game free for the downloading.
I played the heck out of that game back in the day.
Carnage are a set of free wargames rules for auto combat in a slightly different world. The author explains:
Carnage is a racing battle game set in Britain in 2003 or so. Players take control of vehicles from humble Reliant Regals to top-spec Lamborghinis, and then kit them out with improbable Equipment and weaponry, with a view to slaughtering similar minded opponents. They take part in “duels”, (races against each other for honour, pride or cash….but mostly cash), in order to better their machines and skills for the next race. What the end goal is of all this, probably no-one knows, and the Police seem strangely absent from all this. Ah well, who said reality was fun?