The P-47's initial success in combat was primarily due to tactics, using rolls (the P-47 had an excellent roll rate) and energy-saving dive and zoom climbs from high altitude to outmaneuver German fighters. Both the Bf 109 and Fw 190 could, like the Spitfire, out-turn and out-climb the early model P-47s at low-to-medium altitude. Once paddle blade propellers were added to the P-47 in early 1944, climb performance improved significantly. The Thunderbolt was the fastest-diving American aircraft of the war—it could reach speeds of . Some P-47 pilots claimed to have broken the sound barrier, but later research revealed that because of the pressure buildup inside the pitot tube at high speeds, airspeed readings became unpredictably exaggerated. But German pilots gradually learned to avoid diving away from a Thunderbolt. Kurt Bühligen, a high-scoring German fighter ace with 112 victories, recalled: