In today's Daily Mail there was a question about the longest scheduled non-stop flight possible, the answer being Singapore Airlines over the Pacific to Los Angeles, which will take 16 hours. However, the return flight takes 18 hours and 20 minutes. Why the 2h 20m difference?
Almost certainly, the prevailing winds. Flights over the Atlantic also have different timings each way - I think shorter on the westward leg (but if not then its the eastward!)