Premium fuel has a higher octane rating, meaning it resists "knocking" — a small, uncontrolled ignition that can occur in high-compression or turbocharged engines designed to run hotter and more efficiently. If a manufacturer says premium is "required," using regular can trigger the engine's knock sensor to pull back timing and power to protect itself, costing you performance and efficiency even though the car still runs. If it only says premium is "recommended," regular will run safely, just slightly less efficiently — worth checking your specific owner's manual rather than assuming either way.