Which was the fastest piston engined aircraft of World War 2?

Sorry, but the P-51H was never operational in WWII. There were a LOT of fast aircraft in WWII, but most were not operational until AFTER the war. The fastest aircraft of the U.S. arsenal during the war was undoubtedly the Lockheed P-80, and even though it was never used in combat, they DID send a pair to Italy just before the end of the war. They were NOT in squadron service, however. So, that would make the fastest operational prop-driven aircraft the Repuiblic P-47M "Thunderbolt", which was used in combat by the 56th Fighter group to run after the German jets then coming into service. It was basically a P-47N fuselage with the big G.E. CH-5 turbocharger with the big R-2800C engine, but with the small wings of the "D" series. They only built about 150 of them. The "M" was easily capable of between 475-480mph. P-51H, Speed 487 MPH.