Flight Haneda Airport departs from Tokyo Haneda Airport at 08:45 and arrives at Seoul Gimpo International Airport at 11:20.
The flighttime is about 2h 35m. The distance travelled ("as the bird flies"): 1450 Km. So the average speed is roughly 561 Km.
The best day to arrive at Tokyo is Sunday and it would be best to avoid Friday, as prices are on average 135.52% higher than on Sunday. (Average prices, based on 506246 datapoints.)
We have a lot more statistics for flights NH 861 and between Tokyo and Seoul.
