How fast does the propeller on a P51 Mustang spin?

Answer 1

Up to about 2,800 rpm.

Answer 2

Someone said "2,800 RPM" ... the engine could turn that fast (maybe as fast as 3,000 RPM).

BUT...because every major piston/prop warplane of WWII had a reduction gear feature, with an engine turning 3,000 RPMs, the fastest that a propeller on a Mustang with a Merlin engine would turn was around 1,400 RPMs.

A typical Merlin on a P-51 Mustang had a gear ratio of just below 0.48:1. That meant that for every revolution of the engine's crankshaft, the propeller turned less than 1/2 of a revolution.