I've heard that you can aim your gun whenever you want while driving (car shooting section would not be scripted, essentially) so, I'm pretty sure it's gonna be keyboard steering.
I don't think it's that bad though, mouse steering, while being more precise, have other issued (like, not being able to look around you, as mentioned earlier in this topic).
I mean, let's face it, keyboard are not optimized for steering and I don't think there's any magical solution that can fix that problem.