If you aren’t dividing miles by gallons fill up, your not getting that. Tunes mess up the computer calculations
You could be seeing the same fuel mileage in your truck, however there are more variables that can affect the final MPG calculated by the vehicle than just the tune.
1. Driving style and route (hills or elevation change.)
2. Tire size (rolling resistance a function of the tire and the surface itself such as dirt, concrete, or pavement.)
3. Parasitic losses in drive train from overcoming inertia of parts, heat, and noise.
4. Combustion Efficiency (work done on the crankshaft.)
5. Chemical Energy stored in the fuel.
6. Wind resistance.
A few things that get figured into the calculation of MPG's for the tune that are often changed:
1. Injector Data.
2. Tire size as calibrated.
3. Gear ratio as calibrated.
There are many factors that lead into a proper gas mileage reading in modern vehicles, beyond the changes in tuning.
Fuel is a big variable with different blends per season and region, and also ethanol content swinging changes the fuel consumption. The gain in fuel economy in our tune comes from primarily taking advantage of the greater octane content and addressing timing accordingly. Also, some efficiency is gained from changing shift schedules to keep the engine in a more efficient area in the power band.
I am curious, as this is the first I have seen anything but a gain with our tune.