Sounds like currently you have a bit to high of a gear.
Steve has an excellent point that the transmission will shift for you. That changes things slightly
Theoretically could gear it faster than if you had a manual, and let it shift when needed
If a manual with same gears you probably wouldn't shift as much as needed to keep the engine happy and running efficiently.
But even so you want that high gear to be usable most of the time, so if on the straight and level you currently can't accelerate in high gear, then even with an automatic you probably need lower gears.
Assuming you have the power for the speed you want given the shape and weight of vehicle, which I think you should, then do as Steve suggest and let it run in OD most of the time, but downshift on the big hills.
Once you get it setup if you find it doesn't need to downshift for much then you need faster gears and will probably find some more fuel mileage.
If it always downshifts and can't hold high gear then you need slower gears and again will find some fuel mileage.
Can you play with your different ratio choices before you take the jump by temporally swapping rear wheels and tires (smaller ones) to change the effective ratio? calculate here:
http://www.onlineconversion.com/bigger_tires.htmGrigg
