* Driving style: Aggressive acceleration and heavy towing drastically reduce fuel economy. Gentle acceleration and maintaining consistent speeds improve it.
* Vehicle weight: A heavier truck (added accessories, cargo) will get worse fuel mileage.
* Maintenance: Properly maintained engines (clean air filter, properly inflated tires, etc.) will perform better and achieve better fuel economy.
* Terrain: Hilly or mountainous terrain significantly reduces fuel economy.
* Modifications: Performance modifications (chips, exhaust systems) can impact fuel economy, sometimes positively, often negatively.
With all that said, a reasonable expectation for a 2002 Dodge 5.9L Cummins is in the range of 12-17 MPG. However, it's not uncommon to see numbers outside of this range, depending on the factors listed above. Someone meticulously maintaining their truck and driving it conservatively might see the higher end, while someone towing heavy loads frequently might see the lower end or even worse.
It's best to consider this a broad range, and your actual mileage may vary significantly.
