Re: AUX Plug and MPG on a Hybrid Civic
Page 211 of the owners manual:
Auxiliary Input Jack
The auxiliary input jack is underneath the accessory power socket on the front panel. The system will accept auxiliary input from standard audio accessories using a 1/8 inch (3.5 mm) stereo miniplug. When a compatible audio unit is connected to the jack, press the AUX button (AUX icon on models with navigation system) to select it.
As for the mileage the best way to increase it is to slow down and keep the tachometer below 2k RPM as much as possible.