Are you getting any codes (Check engine light)?
When my 02 explorer did this, it was bad 02 sensors (downstream).
I was getting about 8 mpg, replaced the two downstream 02 sensors and I am back up to 16 to 18 mpg.
Good Luck
Pure dumb luck. Unless you either did something else at the same time, the problem was intermittent and happened to go away, or you are confusing the upstream (before the cats) with the downstream (after the cat) sensors.
The downstream sensors (after the cat) have absolutely ZERO effect on fuel economy, as they have no control or effect on anything the ecm does to control the engine. The ONLY purpose of downstream O2 sensors is to determine the efficiency of the cat(s). The computer compares the readings from the post-cat (downstream) o2, with the pre-cat o2, and expects to see a fairly big difference. This tells the the ecm that the cat is doing its job and altering the exhaust chemistry. If it does not see a big enough difference, it sets a Catalyst Efficiency code (P420,etc..). That is ALL the downstream o2 sensor(s) do.
Now the UPSTREAM sensors are different. These, the computer relies on, once in "closed loop", to control air/fuel ratio, which in turn affects emissions and fuel economy. A simple code reader will often lead you in the wrong direction here. It is fairly common for an o2 sensor to have degraded, to the point of negatively impacting fuel economy, long before it is "bad enough" to set a code/CEL. You need to see live sensor data from the o2 sensor in question, and know what to look for, in order to recognize/detect a dying "sluggish" sensor. This will require a higher quality "scanner".
The only other idea I have is remove the battery cables for 20 min to reset the computer (not sure if it will help or not).
Hopefully another member can chime in.
Good luck
DON'T do this; at least not until you have a chance to have the codes read, and written down. Otherwise, you will erase valuable diagnostic information.
You say that your mileage has dropped since retiring. I assume that means your driving habits/traffic patterns have changed as well. Keep in mind, that can also have a huge effect on mpg. If you used to make fewer, and longer trips (daily commute),e specially if most of that was highway vs. more shorter local trips now that would explain a drop in MPG.
You said you changed the plugs. What about the wires? Just because they "look" good doesn't mean they are. Wires degrade over time, especially cheap ones. You might also want to clean the throttle body, as you didn't mention that.
I'm not sure on the 3.0L, but that idle does sound a little high. For the 4.0L, it should be around 700rpm. It doesn't seem like much, but the difference between 700 and 950 idle can make a difference. Look for vacuum leaks.
The other thing is to check the "return" of your throttle body/blade closing. On my Ranger, a weak return spring was causing the throttle to stick partially open. It gradually got worse, and would sometimes hang open at a 1500rpm+ idle. It was one of the "coiled" spring on the linkage itself. I ended up swapping parts between 2 different throttle bodies to get one good one. Just be aware, getting that little coil spring to stay wound up, with the right tension, while you tighten the screw, is a PITA.
Also, noting how many miles you get to a tank isn't an accurate way to gauge mileage. There are too many variables. The right way is to fill the tank, reset the trip odometer, drive till nearly empty, refill to the same level/degree (ie not cramming every drop in one time, and just stopping when the pump clicks off the next time). Note the miles traveled. Divided the miles by the gallons (I go to the 3rd decimal- ex. 15.134 gallons)= your MPG.
As a rough idea, anything over 300 miles to a full tank is probably pretty good though. My 4.0L 4x4, '93 Ranger gets about that, give or take. In my truck, 300 miles to a tank works out to about 20mpg. But, I think you may have a smaller tank, meaning higher MPG for same distance.
Despite being a smaller engine (actually, because of it), I wouldn't expect much better mileage then the 4.0L-OHV. The 3.0L was anemic, barely acceptable motor in a Taurus. Putting one in a truck was a bad idea. Its very under powered; meaning it is always going to be working hard just to move the truck. That uses more gas then a larger engine doing the same work, but more lightly loaded/stressed.
