Airlines already operate their jet fleets at or near the optimum fuel-burn-vs-time point; further reducing speed would actually make them less economical due to

Airlines already operate their jet fleets at or near the optimum fuel-burn-vs-time point; further reducing speed would actually make them less economical due to longer cruise times. There are occasions when crews will dial-in a higher Cost Index ( CI ), which tells the flight management computer to bias towards higher speed rather than lowest fuel consumption. But in general they are instructed to use specific CIs that reflect the airline's costs. With the drop in oil prices some airlines have chosen to raise their default CIs, keeping costs the same whilst slightly increasing speed, whereas others have chosen to keep the CI unchanged and to profit from the additional margin.