* Engine size: A 1998 Chevy dually could have several different engine options (e.g., a gasoline V8 or a diesel V8). Larger engines generally get worse gas mileage.
* Transmission: Automatic or manual transmissions also affect fuel economy.
* Payload and towing: The weight being carried or towed drastically reduces fuel efficiency. A dually is designed for heavy loads, so this is a major factor.
* Driving habits: Aggressive driving significantly reduces mileage.
* Maintenance: Proper maintenance (including tire inflation) improves fuel economy.
* Vehicle condition: A poorly maintained truck will get worse mileage than a well-maintained one.
Therefore, there's no single answer. You might see figures ranging from 8-15 MPG (miles per gallon) in real-world driving conditions. The lower end of that range is more likely with heavy loads or towing, while the higher end might be achievable with a smaller engine, light loads, and careful driving.
To find a more precise estimate, you'd need to specify the engine size and transmission type of the particular 1998 Chevy dually you're interested in. Even then, real-world mileage will vary.
