Miley Cyrus and Arnold's Son -- Proof They're Banging ... Or Just Driving Together

Miley Cyrus and Patrick Schwarzenegger drove home together from an event Wednesday night in Hollywood ... which must mean they're totally doing the nasty, right?!

Miley and Arnold's son were seen leaving a Q&A at the Egyptian Theatre for a new movie produced by Maria Shriver called "Still Alice" ... before getting into a ride and peacing out.

Might seem like thin evidence. .. but Patrick admitted in 2011 Miley was his ultimate celebrity crush -- and it's not like he's bad looking or anything.