If you wish to save fuel, the of course something's got to give. After all one doesn't get something for nothing. The ECO also affects other systems on the car that consume energy via the engine. For example with the ECO button on, the A/C and its blower are reduced in power. To can demonstrate this to yourself easily. Try this. When the A/C is running and the blower is on, try switching the ECO off. You will notice an increase in the speed of the blower.