I tip in the US usually $20-25 for CD depending on whether they allow me intermediate stops

I try to tip on arrival but they're usually going to get foreign currency (dollars in the UAE, or dirhams in India!). We once had a CD driver in HYD who was seriously upset that it took us 2 hours to get out of the airport (visa issue) and demanded 500 INR for his troubles, that was uncomfortable.
I am American (amurrikun) so we feel some guilt when we don't tip...