chelios

if your O2 sensor is of questionable decent, REPLACE THAT sunbich

1987 F-150 XLT 4x4

I see that you have the long-stroke, low RPM 4.9L engine.

Is your F-150 2WD or 4WD?

Do you have an Automatic or Manual transmission?

Does it have Over-Drive?

What is your differential gear Ratio?

Does your truck have an Air Conditioner?

Do you have a lot of hills in your area?

The reason I am asking you these questions is
because they will have an effect on fuel mileage.
But you already know that.

FYI -
My 1987 F-150 4WD LWB 4.9L EFI / C6 automatic transmission w/o OD,
with Air Conditioning and 3.55 gears and 35 x 12.50 radial tires gets 10 MPG
average city/highway driving and there is nothing wrong with it.

It's a 65K mile professionally rebuilt & balanced engine.

It did get 12 MPG average with the smaller 32 x 11.50 BFG tires on it though.

The weight difference between the
32 x 11.50 BFG A/T tire and
35 x 12.50 BFG A/T tire is 40 pounds each.

Power?
The 4.9L / 300ci / Straight Six is a heavy-duty truck engine used in many UPS trucks and Ford Dump Trucks up to 18K GVWR and do not have much top-end power.

Just my 2-cents.