Comment by rockostrich

The bottleneck when it comes to a plane going faster is drag which increases with the square of velocity relative to the air. More drag means the plane has to consume more fuel to stay at its current velocity. So if a plane normally goes 600 mph with no wind then a 100 mph tailwind will allow that plane to go 700 mph relative to the ground to experience the same amount of drag as if it were flying at 600 mph on a day with no wind.