Operating a fleet of say 50 A330s, the daily fuel savings between flying around at MRC vs LRC is over 100t.
LRC burns 1% more than MRC for a given distance right?
If I have my sums right, 100T/day would equate to over $36M per year of fuel savings if fuel cost $1/kg
But based on an assumption of an average of 15 airborne hrs per day per airframe at an average of 5T/hr FF, (optimistic) my pocket calculator gets about 37T savings per day, not 100T.
So on that basis the annual fuel saving would be under $14M - not $36M.
But $14M is enough money to make an operator of 50 aircraft take notice.
It seems that most airlines cruise somewhere between MRC and LRC (closer to LRC) if operating at lowish cost indexes - the fuel saving slightly under LRC seems to make economic sense when other factors are taken into account (hence normal cost index is slightly less than LRC in general).