* Vehicle it's in: The engine's fuel efficiency is heavily influenced by the weight and aerodynamics of the vehicle it's powering. A heavy-duty pickup truck will get far worse fuel economy than a lighter vehicle.
* Application: Is it in a truck used for towing heavy loads? Is it used for city driving or highway driving? Towing significantly reduces fuel economy.
* Driving style: Aggressive driving habits will dramatically reduce fuel economy.
* Maintenance: Proper maintenance, including regular oil changes and air filter replacements, can improve fuel economy.
Therefore, there's no single MPG figure for a 5.9L Cummins. You'll find a wide range of reported MPG, from as low as 8 mpg in heavy-duty towing situations to potentially 15-20 mpg under ideal highway conditions in a lighter vehicle with conservative driving. Expect something in the 10-15 mpg range as a reasonable average under typical use for most applications.
To find a more precise estimate, you should look for fuel economy data specific to the *exact make, model, and year* of the vehicle containing the 5.9L Cummins engine. You can often find this information in owner's manuals, online forums dedicated to that specific vehicle, or websites like the EPA's fueleconomy.gov (though older vehicles might not be comprehensively listed).
