Flight Changi Airport departs from Singapore Changi Airport at 22:35 and arrives at Seoul Gimpo International Airport at 06:00.
The flighttime is about 6h 25m. The distance travelled ("as the bird flies"): 2961 Km. So the average speed is roughly 461 Km.
There is an 1 hour time difference between Singapore and Seoul
The best day to arrive at Singapore is Sunday and it would be best to avoid Friday, as prices are on average 83.54% higher than on Sunday. (Average prices, based on 400472 datapoints.)
We have a lot more statistics for flights KE 644 and between Singapore and Seoul.