The acceleration of gravity is 9.80665 m/s/s. Assuming there is no air resistance, an object will increase its speed by 9.80665 m/s each second. The speed at 6.6 seconds is 9.80665 m/s/s *6.6 seconds=64.72 m/s. Taking two significant figures based on your time, the speed is 65 m/s. The value for gravity decreases with distance from the surface of the Earth; that is why it can vary a bit.
http://www.physic-s.net/2015/11/gravity.html