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All Rangers have tuned exhaust and Cold Air Intakes(CIAs) as stock items.
New spark plugs, and wires(if original) are a good start
Motorcraft or Autolite plugs only on 4.0ls, regular copper or Double Platinum, no Single platinum.
Motorcraft wires, they last about 15 years
Can of Seafoam, or similar, fuel injector cleaner, in the gas tank once a year
New fuel filter once every 5 years.
Upstream O2 sensors every 120,000 miles, they pay for themselves in fuel savings, when they get older engine will run slightly richer.
Throttle cable mod, the throttle cable stretches with age, over time you will lose full throttle.
Remove plastic cover on engine so you can see throttle linkage.
Have someone press down on the gas pedal or put a weight on it
See if you can manually open the throttle more at the engine end of the cable.
If so Google: Ranger throttle cable mod
Just a couple of zip ties
Electric fan conversion
The mechanical fan uses horsepower any time engine is running regardless of engine or radiator temp, it uses more when rad is hotter to move more air.
You can use wrecking yard parts to add an electric fan on the radiator and remove the mechanical fan.
The electric fan does use alternator power when it is on, which uses horsepower, but the electric fan is only on when extra cooling is needed, which is when you are driving slow or stopped, when driving faster the air flow thru the rad does all the cooling needed so fan is off, and you have the extra power.
It isn't much, maybe 3-5HP, but it isn't 0 either :)
Example here: Volvo Electric Cooling Fan
This is one of the only changes that increases horsepower AND increases MPG, those two things RARELY go together, lol.