The second group of girls appease Jasper by trading in Mike's car.
Think about it. With Mike dead, the girls now have in their possession a 100% reinforced death-proof car. In short, it's probably worth quite a bit—beat-up or not. So they bury Mike mafia-style, drive the car to Jasper, hand the keys over to him, make it an effective trade-in, and head off with the Vanishing Point car. Fridge Brilliance, really.