In '02, Motor Trend rated a combined mileage test with a similar vehicle (2wd 4.0L V6) at a mere 16mpg. That was the city rating, so is considered to be average. Curious to know what mileage you net with your ST?
Back to gears: If you drive mostly city mileage, upgrading to a 4.10 or 4.56 (if you run over sized tires) will net better economy as you'll get the vehicle up to speed faster than if you stayed with the stock 3.73/3.55 gears. If you drive a lot of highway and don't have problems now with gear hunting, you could also get a taller gearset. However, that will lower your city mileage and the engine will work harder to get the vehicle up to speed. A good site is Yukon Gear & Axle
to find available ratios and other related parts.
To find the stock ratio, you can do three things:
1. Email Ford and request a factory build sheet, they'll take their sweet time getting it to you (about a month)
2. Take the differential apart and calculate the rotations relative to the driveshaft (can be hard without the proper tools/math skills)
3. (easiest) Look in the glovebox for a sticker that should have the ratio listed on there.