Flight Gimpo International Airport departs from Seoul Gimpo International Airport at 12:05 and arrives at Tokyo Haneda Airport at 14:15.
The flighttime is about 2h 10m. The distance travelled ("as the bird flies"): 1450 Km. So the average speed is roughly 669 Km.
The best day to arrive at Seoul is Saturday and it would be best to avoid Thursday, as prices are on average 7.76% higher than on Saturday. (Average prices, based on 1082754 datapoints.)
We have a lot more statistics for flights JL 92 and between Seoul and Tokyo.