Flight Narita International Airport departs from Tokyo Narita International Airport at 17:10 and arrives at Seoul Incheon International Airport at 19:50.
The flighttime is about 2h 40m. The distance travelled ("as the bird flies"): 1555 Km. So the average speed is roughly 583 Km.
The best day to arrive at Tokyo is Monday and it would be best to avoid Wednesday, as prices are on average 126.25% higher than on Monday. (Average prices, based on 4215234 datapoints.)
We have a lot more statistics for flights KE 712 and between Tokyo and Seoul.
