I routinely override the cruise speed from ECON to LRC to get a higher cruise mach at lower CI'es. For example, with a CI of 12 a typical cruise mach is .76. If I manually select LRC the cruise will pick up to .79.
Personally I don't really get that. The airline obviously has spent time and money organising a system that determines the c i they want you to use on that particular sector. This will be based on their labour costs and their engineering costs and their fuel costs. So why don't you use it? If your airline does 180 average length sectors a week, a 100kg fuel saving on each flight is 18 tonnes a week , 936 tonnes a year. Thats a lot of gas.
Determining cost indexes seems quite complicated, on one flight MRC (c i zero and min fuel burn) can be .745 with a LRC of .778, and the next flight MRC can be .77 with a LRC of .79
On the latter even c i 60 will be slower than LRC.
If you aren't late I can't see any good reason to fly faster than the c i that the company has spent time and money determining.
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I'm paid by the minute but don't like to spend one more minute at work than necessary.
your only reason or is there something else?