If I remember correctly, the 2.3 Duratec engine is supposed to get about 29 mpg but that is mileage on the highway and best mileage is usually seen at speeds around 55-60 mph. Speeds above that, the mpg is going to drop. As far as mpg for the stop and go traffic in town, I have no idea what that would be.
As mentioned above, 3.73 would give the best mpg and performance compromise.
A 3.45/3.55 gearing and 4.10/4.11 gearing are going to give lower mpg.
3.45/3.55 is a "taller" gear, so performance is going to drop. In my opinion and experience, that gearing is too tall for a four cylinder engine. But I live in an area with a lot of hills and valleys in the Appalachian Mountain chain. A flatter area might be better suited.
4.10/4.11 gears are lower, so get up and go is going to be best at the expense of mpg. In my opinion and experience, that is the best match for a four cylinder engine but I've only driven older, less powerful four cylinder trucks. The Duratec engine is more powerful and more efficient.
So, a 3.73:1 axle might work out ok. The truck won't be as peppy but won't be a dog either. But you might gain a few mpgs in the process.
All that being said, we need to know what mpg you are getting in order to say if anything is out of place. Driving style and other details are a need to know as well. Abrupt driving and a heavy foot are going to kill fuel mileage regardless of what vehicle you have.