I hear you!! 10mpg average and 12mpg highway is what I'm getting right now. Before I started having problems, (Burned valve on passenger bank) I was getting 17-18mpg highway with 3.73 gears and 33x12.50/17 Mickey's. About 13mpg in town. I made a trip to Orlando and averaged almost 19mpg, but that was staying strictly at the speed limit and stock tires. I let my son drive it for a couple of years and then parked it due to the burned valve. It stayed parked for about 4 years until I decided to drop a crate motor in and put it back on the road. The dealer that put the motor in was supposed to give me a turn key truck for a daily driver. Fix all the codes, track down the problems, do a bumper to bumper service, and deliver a truck that I could just get in and drive. Long story short, the dealer (Bud Shell Ford in Dexter, MO) wouldn't return my phone calls or emails and so far I've had to replace the brand new (Autozone) battery they installed, rebuild both drive shafts, replace the idler pulley and tension arm, plus all the new stuff I mentioned in my initial post. It's been one heck of a journey, but I'm learning some stuff along the way. I'm sure once I get the EVAP system lined out it will improve some, I just don't know how much.@Hoseman I hate to be the voice of pessimism on your fuel milage. (The EVAP,, good luck, that's solvable if you stay on it.) But 12MPG for the 97-2001 2V engines is in the window. (Mine has always gotten terrible milage, since brand new.) About 11.5 city, and 13.5-14.5 freeway. I have the 4x4 model '99, which is in constant AWD mode always.) Years that have the 2WD switch option do a little better, usually +1 MPG.
I hear people that have gotten what you used to get e.g. 17-18MPG, that is at the VERY high end of the scale, whereas 10-11 is the bottom. Common is GOOD is 13MPG city, and 15MPG freeway. Mine has always done worse. No codes, did have to replace the roller followers on the left (drivers) bank2 block, but basically stock. Its always just been a gas hog. Mine is also, to some extent expected to be on the low side, as I have the tow package...
That is the small tires, the limited slip diff, and the lowest gear ratio. That provided for maximum tow dead-start torque (by Ford), but it costs gas mileage. People getting 13MPG city, and 15-16MPG freeway usually have slightly larger tires, and a higher gear ratio, and most of them have 2WD only models.
I'll update this thread as I journey along and find other issues. I'm sure once I replace the canister and solenoid (hopefully this weekend!!) I'll end up having to drop the tank, new fuel pump, new tank pressure sensor, new vacuum lines, & new vent valve. I guess once I get everything replaced I'll evaluate whether I keep throwing money at it or not. More than likely I will simply because I love the old truck and the newer ones just don't do for me what this one does.