When you next stop at a gas station, you may see multiple fuel grades available. Besides regular unleaded and premium unleaded, there's often a mid-grade option labeled as 'Plus' or something similar.
The octane rating, displayed as a large number on the pump, indicates the fuel's resistance to engine knocking. According to the U.S. Energy Information Administration, 87 octane is regular, 89 or 90 octane is mid-grade, and premium ranges from 91 to 94 octane. Higher numbers mean better knock resistance.
Knocking, also known as premature detonation, happens when fuel burns unevenly in an engine's cylinders. This can cause serious damage if not addressed. High-performance engines often need premium fuel to avoid knocking, while regular engines typically run fine on 87 octane. The 89 octane mid-grade sits in between.
The history of 89 octane unleaded dates back to 1975, when the EPA banned leaded gasoline, leading to a shift to unleaded fuels. Since unleaded gas required more processing, premium unleaded cost more. Gas stations marketed 89 octane as a cheaper alternative to premium, with higher octane than regular. However, it never became very popular, now accounting for only about eight percent of total gasoline sales. No modern vehicles require 89 octane fuel.
If your engine pings or knocks with regular gas, it might be due to low octane fuel, but other issues like carbon deposits, worn spark plugs, a lean air/fuel mixture, bad spark timing, or overheating could also be the cause. It's wise to consult a mechanic to rule out non-fuel-related problems before switching to a higher octane fuel. Otherwise, you might pay more without any benefit.
Despite its low market share, 89 octane gas is still available because it's made by blending high-octane premium with low-octane regular, often right at the pump. Anyone who wants it can purchase it.