* Turbocharged Conversion: A factory 1989 Chevy 2500 didn't come with a 5.0L turbo V8. This is a significant modification. Turbocharging drastically affects fuel economy, depending on the size of the turbo, tuning, and driving habits. It'll likely get *significantly* worse mileage than a stock engine.
* Tire Size: 265/75R16 tires are relatively large for that truck. Larger tires increase rolling resistance, leading to lower gas mileage.
* GVWR (Gross Vehicle Weight Rating): 8600 lbs is a heavy truck. The weight significantly impacts fuel economy.
* Driving Style and Terrain: Aggressive driving, towing, and driving in hilly or mountainous terrain will all drastically reduce fuel economy.
* Maintenance: The condition of the engine, transmission, and other components greatly influences fuel efficiency. A poorly maintained truck will get worse mileage than a well-maintained one.
Instead of a specific MPG number, expect extremely low gas mileage. You're probably looking at 8-12 MPG, maybe even less, under ideal conditions (highway driving, light load, gentle acceleration). In real-world driving scenarios, it could easily be *lower* than 8 MPG.
To get a better idea, you'd need to:
* Find someone with a similar setup: Search online forums dedicated to classic Chevy trucks or turbocharged vehicles. People with similar modifications might share their experiences.
* Consider a fuel efficiency monitor: Installing a device that tracks your MPG in real-time will provide the most accurate data for *your* specific truck.
Keep in mind that this type of modification is likely not fuel-efficient. It's intended for performance, not economy.
