, catalytic converters indirectly reduce fuel consumption. For catalytic converters to work properly they require (as in MUST have) a precise balance of fuel and air called a
Stoichiometric ratio. (Green line below) In a gasoline this means 14.7:1 by mass. (Air mass: Fuel mass). This is the ratio that yields the best (lowest) emissions result.
The best fuel economy ratio is about 16.1/16.2:1, a “lean” mixture that would take you much farther on a gallon of gas. (Purple line below)
Unfortunately Oxides of Nitrogen “NOx”, which enjoy a fairly low value with a properly fed catalytic converter, triple when the engine is tuned for maximum economy.
If one simply removes the catalytic converter, fuel economy won’t change much but if the engine management system is reset without regard to the converter’s needs, then you would get much better fuel economy.