Flight Changi Airport departs from Singapore Changi Airport at 23:55 and arrives at Tokyo Narita International Airport at 08:00.
The flighttime is about 7h 05m. The distance travelled ("as the bird flies"): 4385 Km. So the average speed is roughly 619 Km.
There is an 1 hour time difference between Singapore and Tokyo
The best day to arrive at Singapore is Saturday and it would be best to avoid Friday, as prices are on average 89.1% higher than on Saturday. (Average prices, based on 875804 datapoints.)
We have a lot more statistics for flights SQ 638 and between Singapore and Tokyo.
