| Higher-Octane Share | Resulting Octane |
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Blending Octane at the Pump
Sometimes the grade you want is not on the pump, or you want a specific octane between two grades, such as 91 from a station that only offers 87 and 93. Because octane blends roughly in proportion to volume, you can split a fill-up between two grades to land in the middle. This tool tells you how many gallons of each to put in.
What the Octane Number Means
Octane rating measures a fuel’s resistance to knock, the premature ignition that can damage an engine under load. In the US, the pump number is the average of two test methods, shown as (R+M)/2 on the yellow label. A higher number resists knock better but does not contain more energy or make more power on its own.
Use What Your Engine Needs
Running higher octane than your engine requires is usually wasted money, since the engine cannot take advantage of it unless it is knock-limited or tuned for it. Conversely, running below the required octane can cause knock and trigger the engine to pull timing, hurting performance. Match the manufacturer specification, or blend up if a knock sensor is asking for it.
Frequently Asked Questions
Does octane blend perfectly linearly?
Close enough for practical mixing at the pump. Real blending has tiny non-linearities, but volume-weighted averaging is accurate within a fraction of a point.
Can I make 91 from 87 and 93?
Yes. Roughly two parts 93 to one part 87 lands near 91. The calculator gives the exact split for your tank size.
Will higher octane clean my engine?
Octane itself does not. Detergent additives, which most top-tier fuels include across grades, are what keep injectors and valves clean.