Let look at fuel mileage another way.
All the FI systems work on a feedback loop between software and hardware.
This loop 'performance' is basically set in software by tables. The sensors respond to operating conditions, feedback their signals to the program for comparison to the tables, then hardware sensor output signal corrections are made from those differences, on a full and real time basis to change the engine operation.
What affects this loop performance?
Some but no all and in any and all, in full time combinations;
Truck weight,
Tire pressure,
Drive line losses,
Ox sensor feedback accuracy and response,
Mass Air meter response,
Operating temperature,
Spark plug performance,
Fuel quality,
EGR operation.
..This is all in a fault free system as designed...
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Now add to it the right foot actions and even the left foot use.
The throttle opening directly affects the amount of fuel used to move the weight at any speed. The left foot kills all the right foot actions that were taken.
Bottom line is the total system design sets the average fuel consumption baseline by truck weight, gear ratio, fuel quality and engine design and equipage ect.
Anything that performs less than this level is suspect in any single or combined way.
Since the truck is aged almost none of these parameters could be expected to work at a new level but still within specs.
The system overall control is by a computer program from tables that are used to keep the system within 'practical' limits. When tables shift out of these limits trying to perform a correction, a code is set to tell about it.
This means some have better fuel mileage than others and at or under different conditions.
An attempt to intervene with changes to the design is usually met with a system response as a shift in the operating tables, and 'counters' many kinds of modifications, so you do not see the full expected result for the efforts.
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As an example; suppose fuel pressure is intentionally reduced at the fuel regulator in an attempt to run the motor leaner for improved fuel mileage! No can pull this off. The Ox sensors are designed to run an average air to fuel ratio about 14 to 1 +/- some amount. As soon as the fuel is reduced these sensors react to detect this and feed back their signal as 'Lean' or as excess oxygen in the exhaust. This feedback causes the computer to richen up the fuel for the correction the system is designed to run at from the software tables. this action defeats the attempt to improve fuel mileage via that intended method.
An example of inter action of two systems is the cooling system operating temperature. If too cold, the CHT sensor tells the computer to increase fuel injection amounts from a signal that could be caused by a thermostat stuck open. Result is a loss of fuel mileage but you generally never fell the difference in engine operation.
All the systems operate nearly the same way as an orchestrated total function.
As well, there is inter reaction between most all these systems as one can affect the other's operation.
So it can be seen that making changes on the outside is very often negated by the system unless it is a reduction in rolling resistance, a reduction in throttle opening, a reduction in any parasitic losses or anything that reduces the engine loading. Even then there are still base line limits by the engineered design.
Good luck.