What do the numbers represent in the performance rating of aviation gasoline below grade 100?

In aviation gasoline ratings below grade 100, the numbers primarily indicate the performance characteristics of the fuel in relation to its combustion properties with different air-fuel mixtures, known as the lean and rich mixture performance ratings.

Higher performance fuels are designed to operate optimally under lean or rich air-fuel mixture conditions without causing knocking, which is a detonation issue that can harm engine performance. The numerical rating reflects the fuel's ability to resist detonation when mixed with specific amounts of air.

While the other options touch on different aspects related to aviation fuel, they do not accurately represent the specific meaning of the numerical ratings. For example, the amount of lead in the fuel is relevant for understanding the fuel’s characteristics and compliance with regulations, but it does not directly correlate with the performance rating. Similarly, altitude performance depends on multiple factors in the engine and environment rather than just the fuel rating, and the total volume of the fuel mixture pertains to quantity, not performance characteristics. Thus, option A directly addresses the critical performance metrics tied to fuel mixtures in aviation applications.