I've had my 98 2.5 5 speed for a couple months now and had been getting steady low 20s. Not highway, not city, mostly running around on gravel/side roads at 25-50mph. Was running 235/75-16 tires and 3.45 gears. I was averaging around 250 miles to a tank, but the speedometer wasn't correct because of the tires.

I just filled up after installing new plugs (Motorcraft Platinum) and wires and an 8.8 4.10 spooled rear. I was hoping the mileage was improved, but I got an underwhelming 270 miles on 14 gallons, or around 20mpg. The speedometer is accurate now.

The truck runs just fine, although it's been setting a P0171 since I got it that I haven't been able to figure out. I've looked and looked for a vacuum leak but haven't found one.

What do you guys get out of a tank?

 

If O2 sensor is original change it, just the one by exhaust manifold not the one behind Cat Converter.

P0171 means computer is running the engine Rich because O2 is seeing too much Oxygen in the exhaust(Lean), so MPG will be low
Make sure you don't have an exhaust manifold leak, that sucks in oxygen and O2 will report false Lean.
Older O2's, over 100,000 miles, start to fail Lean, so computer runs engine richer, so worth while to replace O2 sensor when you get over 100k miles, 150k for sure

Also Clean the MAF sensor, always, anytime you have a Lean or Rich code

 

I replaced the pre-cat 02 a couple months ago. Exhaust is quiet and leak free.

I haven't messed with the MAF, but I will clean it just to see what happens.