Rusty,
For optimal fuel economy with 3:73's I would use 28.5" (28"-29").
You do have a STX, does it happen to be a Hi-Rider; only STX trucks came with the Hi Rider option, the fact that your running 31's suggests to me that you may have a Hi Rider, 2" factory suspension lift ..)
In regards to inexpensive ways to improve efficiency:
*full synthetic fluids; oil, gear oil + tranny
*1 or 2 heat ranges hotter spark plug
*get a 86-87 throttle body
*8mm or larger Si supresion plug wires
*drop in K&N or equivilent air filter
*a complete tune up
*uniform, optimal tire pressure
*correctly torqued lug nuts
*drive conservatively
All of those items will contribute to the highest possible fuel economy but, depending on your budget here are more ways to increase your 2.9L's efficiency:
A. remove the crank shaft driven fan clutch for an electric fan. While this does not improve power, it makes an appreciable reduction in the load on the 2.9L.
B. unless recently performed your engine could benefit from a quality multi angle valve job, even without the full valve job, replace the valve guide seals and determine whether you valve guides are working well.
C. disconnect your a/c belt
D. pull out your rocker arm shafts, tear them down to their separate components, thoroughly clean, increase the size of the oiling holes in the rockers and shafts.
E. correctly set hydraulic lifter preload.
F. if your engine is ready to be rebuilt:
*at this point you will need to decide how much cheap is and whether you want to invest in efficiency improvement or a monthly auto payment.
- Ford 2.8L flat top pistons; they will increase the comp/ratio by a whole point +4hp
- flow bench port/polish heads to intakes +20hp
-custom reground cam +20hp
- quality headers +20hp
- appropriate increased size in exhaust tubing with free-est flowing components
- bore out your T/B or even a tapper hone +2hp throughout rpm band
- digital or CD multi spark ignition
- a computer chip to help with timing
- advance your timing to max vacuum us read; this is where YOUR engine will be MOST efficeint.
All these increases in efficiency are there for the taking, some cost more but also yield more