Econ Climb is based on weight & Cost Index. CI is a relationship between fuel costs and time-based (maint, engs, deprec etc, maybe lease also) costs:
More fuel = less time
Less fuel = more time
Econ Climb is based on your operator's cost structure, whereas Best Rate-Of-Climb is based on aerodynamic efficiency based on your performance manual.
Would pay to read your Perf Man, also, if Boeing, there is valuable information in the FCTM (Flight Crew Training Manual).