Definition
A grade of aviation gasoline (avgas) with a minimum lean-mixture octane rating of 80 and a rich-mixture performance number of 87, historically dyed red and used in low-compression piston aircraft engines. It has now been largely replaced in service by 100LL.
Plain English
An older, lower-octane type of aviation fuel made for engines that don't need high-octane gas. The two numbers describe how well the fuel resists knocking when the engine is running lean and when it's running rich.
Context Anchor
Seen in fuel-grade discussions, aircraft manuals, and fuel placards for airplanes approved to use this grade of aviation gasoline.
Derivation
The numbers come from the octane rating system used for piston-engine fuels. The first number (80) is the lean-mixture octane rating; the second (87) is the rich-mixture performance number. Higher numbers indicate greater resistance to detonation (engine knock).
Why Pilots Care
Using the correct fuel grade prevents detonation, engine damage, and loss of power during flight.
Intuition Check
80/87 is not a fraction, a percentage, or a mix ratio. It is two separate fuel performance ratings for two different engine operating conditions.
Example Sentence 1
Older training aircraft with low-compression engines were originally certified to run on 80/87 avgas.
Example Sentence 2
The pilot declined to add automotive gasoline and waited for a delivery of 80/87 instead.