What I liked about it was that it wasn't too laboured. The flow was constant and there were some good lines like:
MIKE:
How is he anyway?
JOHN:
Being sick. Well, not really being sick. It's like he's leaking from the mouth.
MIKE:
I'm pulling over.
JOHN:
You suddenly care?
MIKE:
No, I'm hungry. There's a burger bar up there.
The only thing I would say is that although I immediately took to the characters, they all seemed too similar. When you go back to this (as I really hope you will), try and flesh them out and that way it should become even funnier. Well done though.