Refer to figure.
Maximum range occurs when you get the greatest distance (D) for a given amount of fuel (F). The specific range is equal to TAS/DRAG x 1/SFC, where TAS/DRAG expresses the requirement to operate at the greatest forward speed for the least amount of drag.
All aircraft produce thrust to overcome the drag of the aircraft. Thrust required to maintain an aircraft in steady level flight equals drag. Therefore, it is the drag curve. Where the ratio of thrust required (drag) to TAS is a minimum, we will have the maximum range speed.
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