Just a bit of info to clarify whats already been said..

Generally, Closed loop is not enabled during medium to heavy loads so in effect is turned off and reverts to the original mapping.

Closed loop is enabled when ALL the following conditions are met...

(rough figures)

Engine temperature above 55 degrees C
Below 3800 rpm
Below 5psi
Below 60% throttle position
Steady state RPM for 0.3 seconds
Steady State manifold pressure for 0.5 seconds

So you can see any sudden load changes or demand for power will turn closed loop off.

If you drive it gently without sudden throttle movements it will reduce fuel consumption.
If you rag it all the time even short bursts of medium acceleration, it will still drink fuel.
Dont forget, it is a performance car...LOL

If you want to save the planet, buy a push bike