While Dodge diesel trucks of that era *can* achieve relatively good fuel economy for their size and power, 21 mpg is on the higher end. Factors influencing fuel economy include:
* Engine condition: A well-maintained engine, properly tuned and with no leaks, will perform better.
* Driving style: Gentle acceleration, consistent speeds, and avoiding excessive idling significantly impact fuel economy. Highway driving generally yields better mpg than city driving.
* Vehicle weight: A heavily loaded truck will get significantly worse fuel economy.
* Tire pressure: Properly inflated tires are crucial for optimal fuel efficiency.
* Aerodynamics: While less of a factor in a truck, anything that reduces wind resistance helps.
* Modifications: Performance modifications (chips, larger injectors etc.) generally *reduce* fuel economy, while some aerodynamic or efficiency modifications might slightly improve it.
In short: While not impossible, a consistently achieved 21 mpg in a 2004 Dodge diesel pickup would suggest either a very lightly loaded truck driven meticulously on the highway, or possibly some form of modification optimized for fuel efficiency (though these often come at the cost of power). A more realistic expectation for that era truck under typical use would be in the range of 12-18 mpg.
