The Cost Index is a management tool. The rw value can change based on price of fuel, aircraft, stage length, crew, etc. From an operating standpoint, the higher the CI the higher the speed and frequently lower cruise altitude as "optimum".
Most current airframes will see little cruise speed change with different CI's, but the more noticeable change is in climb/descent speed.
As a rule of thumb, lower CI with tailwind, higher with headwind; 30-50 is a good range. Typically, 199, or less, is the max value to enter and will keep it "on the barber pole".
Cheers!
Sean
