It should be no mystery that a dashboard mileage readout is showing significant gains in MPG when tires are inflated to higher PSI's. Think about it; it is calculating that the engine isn't having to work as hard.
The readout will indicate a lower MPG if you are going up a grade (engine working harder).
It will indicate a lower MPG if you have more people in the vehicle.
Likewise if you are driving into a strong headwind.
Neither of our cars have this readout, but I have seen one on TV.
