a manual gets better gas mileage because 6th is a ratio of .5 to 1. In the auto, 4th is .76 to 1. THAT is what makes the manual get better gas mileage. Whoever told you having better gas mileage means having the engine at a higher rpm, have them drive around at 6000rpm in 1st for 30 minutes and see how much gas they go through. The LOWER your rpm's are, the better gas mileage you will get.
Your best bet is to leave the car in OD, and use as little gas as possible when driving. Sure its hard not to floor it, but with light pressure, you will get MUCH better gas mileage.