Please make a signature to help us out with questions like this. Plus, I would just leave the stock gears in there, there's no reason to swap to different gears if you have stock tires and don't off-road.
to get best mpg. a 3.55 would be best.. most likely you have a 3.73 or 4.10 already.. but i am with Brenton no need to go changing it. if you wanted MPG you bought the wrong vehicle a pruis would have been a better choice