I bought a 2004 Ford Ranger about 6 months ago. It was immaculate with 117K miles on it.
For the first 3 or 4 months i was getting 19.5 miles per gallon consistently. My driving is 85
percent hwy and is on the same hwy with very little elevation change.

The last couple of months my fuel mileage started to decline. I went online and started to read
the causes of declining fuel mileage. The truck was running the same as before the decline.
There were no engine codes. I spent 30 days bringing myself up to speed on the possible causes.
I tested my O2 sensors, changed spark plugs [ the old ones were in excellent shape and burning clean],
I changed the air filter [ the old one was clean] , I charted long and short term fuel trim [they were within
the norms].
I did 4 things in one session that brought my fuel mileage back to normal. I changed spark plugs, the air
filter and disconnected the battery for a half hour after having read a couple articles that indicated that this
would reset the PMC and lastly i checked tire pressure. The tire pressure was 30 psi, i had read articles by
a couple of people who said low tire pressure had improved mileage considerably. But!!! The consensus of
online articles i read was that 6 llbs of air pressure might account for 1or 2% of the mpg. I had lost a total
of 3 plus mpg before i started to try to track down the lost mileage.

My conclusion was it had to tires or the PMC. I drive in the same conditions , usually 55mph for 35 miles and
then a couple miles in town. I was thinking that maybe the fact that i never put my foot in it that it could have
locked the computer in a richer fuel mode.

Any intelligent opinions???!!!