Man convinces Sheriff he's a great drummer even though he's not! Late 70's comedy-drama?

Ok so I think most of the film focused on a band but what I do know is they're playing a gig and their instruments have been left behind or something.

This young black dude has been assigned the task of getting the instruments to the venue in time and is driving his van across state/country. He gets pulled over by the cops and I think it's a stereotypical 'southern' sheriff and the driver is freaking out. He claims that he's a musician so the sheriff says, go on play us something. He then does the worst drumming imaginable! After a pause the sheriff is like, boy that's amazing and the driver is like wtf how did I get away with that!

I remember that the instruments made it to the band in time for the show. I think at the end of the movie people were dancing in the streets outside the venue, almost like it was a musical.

Had to be late 70's or early 80's. Ring any bells?