As an engine accumulates miles, its octane requirement increases, sometimes to the point of needing a higher octane grade than that initially recommended for the engine. We know that as engines accumulate miles, their octane requirement increases, and often this increase does not exceed the octane quality for which the engine was designed. But if the engine begins to ping or knock when accelerated hard, climbing a grade, or towing a trailer, the next higher octane grade will likely satisfy the engine's new octane requirement. Studies have shown that about 60 percent of all vehicles are satisfied by regular grade gasoline. Midgrade satisfies about 80 percent, and premium nearly all engines.
Many modern engines have control systems that keep the engine from knocking.
Some of these engines can benefit from using higher than recommended octane grade gasoline. Many of today's engines have knock control systems. These systems are designed to sense knock before you can hear it, and adjust spark timing to eliminate the knock. If the gasoline used does not satisfy the engine's octane requirement, the system will retard spark timing. Retarding spark timing causes some loss in fuel economy, and reduces maximum power.