Never changed my 67 Barracuda.Because my first 2 trucks I never changed it. Over 150k miles on each. We never changed our work trucks. We never changed our E350 vans, fully loaded, well over 200k miles. Except for the one I broke a axel on. Never changed on my 65, 67 Impalas. Or 73 Caprice. Or 66 and 75 Dodge Darts Never had a differential fail. But have learned it was bad practice.