Next week, on the way home, start a political argument. Get into a shouting match with Mrs. Lib.
When she demands that you get out and walk, defend yourself with "moral isolationism" -- she can't make a moral judgment about your culture.
Of course, if she doesn't practice what she preaches, you'd be risking your ride. And we've learned that being a liberal is a license to preach without an obligation to practice...