@ phiggsbroadsband, post just above :
Cost Index is indeed based on Cost time/ Cost Fuel ratio , with values not necessarly updated by company . Some companies have not updated their CI for years. But they adjust basic CI to their fuel /speed policy .
Pilots just adjust the CI of FMS, in flight to adapt needed time of arrival . FMS speed will ajust accordingly.
CI = 0 gives the maxi range speed.