Miley Cyrus -- Bad Ass B-Day Gift From Billy Ray

Billy Ray Cyrus just won a bunch of cool points with his daughter Miley Cyrus ... by dropping a TON of cash on a new set of wheels for her 21st birthday.

Sources close to the deal tell us Billy Ray plunked down $24k on a 2013 Can Am Spyder ... with a customized Miley Cyrus flair.

If you look closely ... it's not a Gucci design -- it's got tiny M.C.'s all over it.