Based on the OBD standard, MPG is calculated using a combination of constants
(air/fuel ratio, fuel density, etc) mass air flow (MAF) and speed.

The problem with the OBD standard is that it groups the gas engine and diesel
engine together.  However, since the combustion ratio of gasoline is
different than diesel, the MPG comes out to be incorrect on diesel.

The MPG reading on diesel engines will be off by a factor of approximately 3.5 So if you’re
getting 5 MPG reading on a diesel engine, it means the true MPG is approximately 17.5. (3.5 X 5).

Unfortunately, the Xirgo unit does not have logic (YET) that determines what type of
engine is in the vehicle.