The three variables of cost index are:
1). Crew costs per hour
2). Maintenance costs per hour
3). Fuel costs
If you fly LRC your speed will reduce as your weight reduces. But you will get more MPG than any cost index value.
In the Gulf they used to fly around at max mach cos fuel was zip. ( high CI )
In Europe most 737 operators who don't care too much about time constraints fly between 30 - 40.
Just be careful though if you are behind THY who fly slowly cos they are paid by the minute. 160 kts at 20 nms is the norm!!