"Greetings,slower speed means more time in the sky, means longer flights, means Cost increase such as maintenance, flight crew, over flying taxes, head wind ( the longer you stay in it the more you suffer from it) You were being asked about FUEL SAVINGS: "Why do you save fuel going slowly like 265 knots in descent ? The aerodynmics behind it are ? I knew it once but have forgotten and am too lazy to look it up.Thanks for helping me out."Going slow will surely save you fuel but will cost you in terms of time. This is the concept of COST INDEX.