Thank you for the replies guys,
Old Smokey,
that makes sense, so according to this graph
Airspeed
At 240 KIAS at FL 160 and FL 240 gives a difference = drag of around 3 kts which means then it would not have very significant influence on fuel consumption ?
Deltabravowhiskey,
you are travelling more miles per unit of fuel consumed, but for example if you need to maintain a constant IAS during holding you don't take into the account how many miles per unit of fuel you are travelling.