There is no consistent cruise speed used on any flight, regardless of distance. Cruise speeds often reflect traffic patterns in addition to distance. To come up with an average speed is quite difficult. However, most airliners cruise between 400-500 knots. Cruise speed is assigned before the pilots even begin their pre-flight checks. A group of dispatch folks perform all the calculations needed to make the flight most economical according to its range, assigned altitude, and estimated traffic patterns. This speed can be changed by the pilots, should they request it for one reason or another. Air traffic control also adjusts speeds as needed. In climbing from the airport, pilots obey a profile of different assigned speeds, headings, and altitudes. Once reaching cruise altitude, they set the assigned speed and other information into the autoflight system. The cruising altitude will also vary according to flight distance, type of aircraft, and other traffic in the air. The reason I refer to other traffic is because our airspace is becoming increasingly more crowded. Planes may be following each other in line at the minimum separation. If one jet suddenly increases his speed by 30 knots, he will soon violate the minimum separation distance.