According to Wikipedia
In the standard atmosphere:
T0 is 273.15 K (= 0 °C = 32 °F), giving a theoretical value of 331.3 m·s−1 (= 1086.9 ft/s = 1193 km·h−1 = 741.1 mph = 644.0 knots). Values ranging from 331.3-331.6 may be found in reference literature, however.
T20 is 293.15 K (= 20 °C = 68 °F), giving a value of 343.2 m·s−1 (= 1126.0 ft/s = 1236 km·h−1 = 767.8 mph = 667.2 knots).
T25 is 298.15 K (= 25 °C = 77 °F), giving a value of 346.1 m·s−1 (= 1135.6 ft/s = 1246 km·h−1 = 774.3 mph = 672.8 knots).
