Also, I don't know if maybe this is crystal clear to everyone involved, but I haven't seen it mentioned. Every car, even "normal unmodified" cars, have the stats: power, looks, speed and weakness. It's just, you only use them if you have A no shit driver.
You could of course add a custom move, like, when you're in a car chase or something, that reads roll+speed.
Maybe you want to add "when someone else tries to help or interfere with you, add your car's weakness to their roll" for everyone. Otherwise taking A no shit driver means you're suddenly worse at helping others from behind the wheel than you were before (which kind of makes sense if you want it too, I guess?).
But if you don't do either of those, and for as long as no-one takes A no shit driver as an advance, you don't need to actually stat the cars up. But be sure to establish them fictionally: is this car actually a powerhouse, rickety, reliable, really fast, eerily quiet, etc.? And with the savvyhead, if they mount a grenade launcher on the car, then they can drive around and, you know, fire grenade shells from the car! That's awesome. Treat it like you would any player firing grenade shells while in a moving car. There are stats for the grenade launcher, if you need its harm rating.