"a Chariot is
always treated as a single model"
"Invulnerable saves are different to armour saves because they may always be
taken whenever the model suffers a Wound or, in the case of vehicles, suffers
a penetrating or glancing hit – the Armour Piercing value of attacking
weapons has no effect on an invulnerable save. Even if a Wound, penetrating
hit or glancing hit ignores all armour saves, an invulnerable saving throw can
still be taken."
Armor save also refers to "model." So, does the chariot benefit from a save/invuln save of the Overlord?