...and then the wind comes into the equation. Generally, in a headwind, itīs reasonable to fly faster for maximum range as well as for minimum time, while in a tailwind, reducing IAS is advisable.
Assume a Piper Cub, cruising at economic 60kts against a headwind of the same 60kts - it will burn lots of fuel but will not make a knot of groundspeed. Opening the throttle a notch and increasing speed to a theoretically less economic 70kts will cost more fuel per hour but make the aircraft slowly gain miles and eventually reach its destination. Now assume the 60kts of wind coming from the rear and pushing the aircraft: in this case, flying at normal cruise speed will see You at the destination in no time, but you can save tremendous amounts of fuel by just throttling back, reducing to maximum endurance airspeed and letting the wind do the work for you. And with an airliner, itīs not really different, a smart choice of flight level and climb/descent schedule vs. expected wind situation can result in saving much cost and time.