What about 'Midnight Run' (1988)?
Am I the only one who thinks that Martin Brest's "Midnight Run" (1988), with Robert de Niro and Charles Grodin, is unbelievably funnier than "Due Date"?
The story is admittedly different, but it also relies on two mismatched characters stuck in a car trip across USA. In my humble opinion, the couple de Niro/Grodin works far better than RDJr/Galifianakis. Yaphet Kotto, Dennis Farina and John Ashton also do great performances. And the script is better written.