* Specific engine: Big-block Chevys encompass a wide range of engines (e.g., 396, 402, 427, 454), each with different displacements and performance characteristics. Larger displacement generally means lower fuel economy.
* Vehicle application: The vehicle the engine is in (e.g., a heavy-duty truck, a muscle car, a boat) greatly affects fuel efficiency. A heavy vehicle will always get worse gas mileage than a lighter one.
* Modifications: Performance modifications (e.g., larger carburetor, high-performance camshaft, upgraded exhaust) significantly reduce fuel economy. Stock engines will generally be more efficient than modified ones.
* Driving style: Aggressive driving habits (rapid acceleration, high speeds) drastically decrease fuel economy. Gentle acceleration and maintaining a consistent speed will improve mileage.
* Maintenance: Proper engine tuning and maintenance (e.g., properly adjusted carburetor, clean air filter) will improve fuel efficiency.
In general, expect very low gas mileage from a big-block Chevy. You're unlikely to see more than 8-12 miles per gallon (mpg), and that's under ideal conditions with a relatively light vehicle and conservative driving. In many real-world situations, it could be significantly less, perhaps even 5-7 mpg. Anything above 15 mpg would be considered exceptionally good. Modern vehicles achieve far better fuel economy.
