"Eat Beans, America Needs the Gas."

"If you don't like the way I drive, get off the sidewalk."

And, what would probably be Madman and Off2cjb's favorite (and I think it is kinda funny)...

"I used to be a Democrat. Then I got a job."

I've thought about getting one that says,

"I used to be a Republican, but then I got a conscience." [Finger]
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"Nature has constituted utility to man the standard and test of virtue. Men living in different countries, under different circumstances, different habits and regimens, may have different utilities; the same act, therefore, may be useful and consequently virtuous in one country which is injurious and vicious in another differently circumstanced" - Thomas Jefferson, moral relativist