Boosting gas mileage in 98 ranger with 4.0L...

Aftermarket: Go to the local supermarket and buy a dozen eggs. Throw out 11 of them, place the remaining one between your right foot and the accelerator. There's very little else you can add on besides a tonneau cover that might have a positive effect on mileage, but I suspect the difference is quite small for what a cover costs, and would take a good while to pay off.

Non-aftermarket: Use the thinnest oil recommended, keep the air filter clean, the wheels aligned, the tire pressures at the maximum recommended value, make sure the brakes are properly maintained to prevent drag, and keep your speed (and acceleration) under control. Try to minimize your use of the brakes, accelerate slowly and never accelerate toward a red light- back off and coast toward it. You'll be amazed at how simply driving like an old man will increase mileage.

There really isn't anything that will make such a difference that you'll get an overall cost savings or ROI within the life of the vehicle. For example, let's do the math...

At 12k miles per year, 18 MPG, and $2.65 p/gal = total yearly fuel of $1767. With a 10% increase in MPG, you save $160.

I don't think you'll find any $160 or less aftermarket solutions that provides a 10% increase in MPG.

Or, some other figures to contemplate...

adding a cold air intake system, headers, and high flow exhaust system will cost about $1000. With that you may, and I reiterate "may", get a

10% increase...as long as you keep your foot out of it...because you'll also get a performance gain that may be hard to resist. To recover the $1000 in the 1st year you'd need a %130 increase (equal to about 41 MPG). Or, going back to only the 10% increase, you'd need 6.25 years just to break even...are you going to keep your 1998 for another 6 years?

Granted, the more miles you drive per year and how often and what you tow and carry will affect the numbers.

For me, I drive, carry, and tow alot. I add aftermarket enhancements for performance, which in a small way increase my MPG. But, I'm more concerned that I can get up a hill, merge into traffic, or pull out of my driveway.

FWIW, be leery when a manufacturer makes an MPG increase claim. For example a claim of 3-4 MPG increase means your 18 MPG truck would need to be 17-23% more efficient...kinda unlikely. When looking at claims, if the manufacturer can't provide exact figures for your vehicle, then percentage increases provide a better measurement than flat MPG number increases.

This may not have been the answer you were looking for, but hope it helps anyway.

There isn't much you can do. When it's time for tires, getting the right tires may net a small savings.

Bill M