Hi, Broadly speaking the engine ECU looks at a number of inputs to decide the fueling to the engine. A few off the top of my air temperature, air volume, throttle position and the difference between the reading from the O2 sensors before and after the catalytic converter. If the CAT is removed, both the readings from the two O2 sensors will have equal value preventing the engine ECU to go into closed loop status and the check engine light will come on. So your question was about fuel economy. As per my very limited understand of modern engines, the car will run rich if it does not go to closed loop. I am open to correction.