Mel Gibson -- I've Got One, HUGE Tip for Ya ...

It pays to service Mel Gibson ... especially in Arizona ... where the actor dropped a massive tip after a fancy Italian dinner Monday night ... TMZ has learned.

Gibson hit up the Bravo Bistro in Scottsdale ... where he dined on shrimp cocktail and osso bucco with the owner of the restaurant and two women.

After 3 hours, Mel picked up the $170 bill ... and left a $150 tip ... that's 88% for all you math-a-magicians out there.