Definition
A grade of aviation gasoline (avgas) identified by its two octane (performance) ratings: 100 when the engine is running at lean mixture and 130 when running at rich mixture. The two numbers describe the fuel's resistance to detonation under different operating conditions. 100/130 is dyed green and was historically a common piston-engine aviation fuel, though it has largely been replaced by 100LL (low lead) in current operations.
Plain English
A type of aviation gasoline rated for high-power piston engines. The two numbers are anti-knock ratings -- one for cruise (lean) mixtures, one for full-power (rich) mixtures. It is green-dyed and is the older counterpart to today's more common 100LL fuel.
Context Anchor
Seen in fuel and oil discussions, fuel placards, aircraft manuals, and older references to aviation gasoline grades.
Derivation
The two numbers come from the octane rating system used for piston engines. Higher numbers mean greater resistance to detonation (uncontrolled combustion that can damage an engine). Two ratings are given because the same fuel performs differently depending on whether the fuel-air mixture is lean or rich.
Why Pilots Care
Selecting the correct fuel grade prevents engine damage from detonation and ensures the engine delivers its designed performance.
Intuition Check
100/130 is not a mixture ratio and does not mean 100 parts of one fuel mixed with 130 parts of another. It is a two-number fuel grade rating.
Example Sentence 1
The older fuel placard near the filler cap listed 100/130 as the approved grade, though the airport now only stocks 100LL.
Example Sentence 2
The engine manual specifies 100/130 fuel for all operations to maintain proper detonation margins.