You could play with this steering constants in here :
//sounds
this.load_sound("mon_start.ogg");
this.load_sound("mon_idle.ogg");
this.add_sound_emitter("carpaint");
return {mass:2315, steering:2.3, steering_ecf:30, centering:2.3, centering_ecf: 40,
com:{z:-0.7}};
}
I see the wheel-radius is 0.3 meters, how far are the centers of the wheels apart on one of the sides ? Also the slip on wheel parameters may have some effect too, doe didn't try to speculate with it looking at the turn-radius. Trying just four wheels instead of six will have some effect too .... just try those things and pick what will work for ya.