FMC er
Don't forget that (at least on the boeings) Cost index is also adjusted by the company, as are optimum and maximum flight altitudes. So you may not be comparing apples to apples. It depends on how the rotating components are paid for compared to the price of fuel. Also different engine installations get different ratings for the cost index. Even within the same engine you can have different cores ie Rolls Royce G and H cores which have slightly different preformance characteristics.
The bigest determinate of speed is still weight. In the 744 at 385tonnes (assuming max take off wieght) optimum is around FL280 and at CI 80, (nil wind ISA) it is around .85 and gets slower as you get lighter.
cheers
FG