Complex sentence: The speedgun in Hiroshima Municipal Stadium records as a 67 mph (107 kmh) slowball, but media in Japan found that the speed was mistaken and should be much slower and they estimated the speed was only 30 mph (48 kmh) after rough calculations.
Split into simpler sentences: The speedgun in Hiroshima Municipal Stadium records as a 67 mph (107 kmh) slowball, but media in Japan found that the speed was mistaken, which should be much slower. They estimated the speed was only 30 mph (48 kmh) after rough calculations.