The Cost Index variations are one thing but more importantly, saving fuel is achieving the optimum level and a favouable postion in the traffic flow for descent. I have made more fuel by getting underneath and ahead of slower traffic to then get my optimum level and a clear path than slowing down to a crawl. Too much emphasize on CI at the expense of airmanship and good planning costs fuel too. At .63 you might as well land somewhere and put them on the train.