"A pint is a pound the world around", that's for water weight

8 pints in a gallon
Gas weights a bit less that water, but still close to 8lbs per gallon.
10 gallons is 80lbs
15 gallons 120lbs
So a full tank of gas is like carrying around a hitch hiker, lol.
Let the fuel trucks carry gas across town for you, get just what you need.
Or don't complain about the MPG, lol.
Since you can't reduce engine size, reducing the weight is about all you can do.
Making the engine run at it's most efficient is assumed already done.
Thinner tires with full air reduces rolling resistance.
Always drive downhill and with the wind, you may not get where you want to go but where ever you do end up you will have gotten there with the best MPG
Yes the tailgate thing is a myth.
My '94 supercab with 4.0l canopy and hundreds of lbs of tools in the back, still gets me around at about 14mpg mixed, when empty I flirt with 18mpg mostly highway