My guide to differential maintenance on a classic American muscle car, because this is the component most commonly forgotten in restoration planning and most commonly the source of noise and failure.

The original rear axles on most muscle cars are open differentials — they send power to the wheel with the least resistance, which is the wrong wheel in most situations. The factory limited-slip options (Positraction, Sure-Grip, Trak-Lok) are the correct choice for any car that will see performance use.

Fluid change: the friction modifier in limited-slip differentials depletes over time and causes chatter. The correct service interval is every two years or 30,000 miles. Use the correct GL-5 gear oil with the friction modifier additive specified for limited-slip units. Standard GL-5 without friction modifier will cause chatter and accelerated clutch pack wear.