A bit of propeller trivia. Working on Navy radials I was intrigued with the engine internals, especially the master rod setup but also the reduction gearing. One of the classes had told me that the tip speed on the prop was kept below Mach1. Later I learned that if the tip speed goes supersonic the shock wave splits the air from the prop surface, destroying efficiency - kind of like the part that's supersonic becomes ineffective. The thing is, the prop speed is actually the Helical Tip Speed, made up of the rotating speed plus the forward component of the aircraft speed. That means the faster the design air speed, the slower the prop has to be, but the bite can be increased to use the engine power. I saw an example where a guy was flying a high performance prop plane cross country and was running behind schedule. He decided to push his engine speed to make up time, going from 2400rpm optimum cruise to 2700rpm WOT and was surprised to see his air speed drop by 15 knots! He backed out to 2400, and regained his air speed. The magic number is 0.85 Mach1 speed for the Helical Tip Speed. Any faster efficiency falls off.

It would be interesting to see how the 80% Mustang accomplishes their 2.13 to one reduction, and also if they had to accommodate two plugs per cylinder which is a standard aviation engine feature.