Originally Posted by Zeeper
It's better to downshift than to lug the engine. Any serious hill with AC on also requires downshifting.
The engine and transmission are geared to make power in the lower three gears, 4th is ok for higher speeds but 5th is overdrive and if you try driving up a big hill in 5th with your AC and cruise on (or a lot of weight like you described) you will notice that the car cannot maintain its speed in 5th gear.
I find the 5th has trouble maintaining speed in general without giving it a decent amount of gas which ruins the purpose of the better mpg