At a fixed weight, the following chart shows that the minimum fuel occurs at a descent speed of 240kias to 280kias, dependant on flight level.  However there is a significant time penalty at these speeds.

Note that the effect of the descent Mach number is very dependant on cruise flight level and descent speed, but is relatively small compared to the descent speed effect, and is not fully investigated here.
These descent charts are typical of the other Airbus aircraft. Generally they show a minimum fuel speed of 260 to 280 kts for flight level 310, reducing to 240kts for flight level 390. The exceptions are the A318, A319, A320 and A330, which show the minimum fuel at 240kias for all flight levels which is slightly lower than the other aircraft at FL310.
Appendix B presents some examples of these descent charts for other Airbus aircraft. 
  The following tables show, for various Airbus aircraft, the descent time and fuel variations for a fixed distance, from FL 350, relative to a 300kias reference speed.
Type
 .Fuel – kg
 
 
 240KT
 260 KT
 280 KT
 300 KT
 320 KT
 330/340KT*
 
A300
 -55
 -60
 -30
 0
 25
 35
 
A310
 -55
 -60
 -30
 0
 25
 40
 
A318, 319, 320
 -50
 -40
 -20
 0
 20
 25
 
A321
 -35
 -40
 -20
 0
 20
 35
 
A330
 -110
 -105
 -60
 0
 50
 70
 
A340–200/300
 -70
 -90
 -50
 0
 50
 75
 
A340-500/600
 -125
 -130
 -70
 0
 70
 100
 

 
 
Type
 .Time – minutes
 
240 KT
 260 KT
 280 KT
 300 KT
 320 KT
 330/340KT*
 
A300
 2.7
 1.5
 0.6
 0
 -0.4
 -0.6
 
A310
 2.4
 1.4
 0.6
 0
 -0.4
 -0.6
 
A320 family
 2.6
 1.4
 0.6
 0
 -0.4
 -0.6
 
A330
 3.5
 2.0
 0.8
 0
 -0.6
 -0.8
 
A340–200/300
 3.2
　
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