* Vehicle weight: A heavier car with the 305 will get worse gas mileage than a lighter one.
* Transmission: An automatic transmission generally reduces fuel economy compared to a manual.
* Rear axle ratio: A higher numerical rear axle ratio (e.g., 3.73) will sacrifice fuel economy for acceleration, while a lower ratio (e.g., 2.73) will improve fuel economy but reduce acceleration.
* Tire size: Larger tires increase rolling resistance, reducing fuel economy.
* Driving style: Aggressive acceleration and high speeds dramatically decrease fuel economy.
* Vehicle condition: A poorly tuned engine or problems with the emissions system will reduce fuel economy.
* Aerodynamics: The shape of the vehicle significantly impacts fuel economy.
A 305 cubic inch small-block Chevy engine in its original application (typically a mid-size car or light truck from the late 70s through the 80s) might have achieved anywhere from 10 to 18 miles per gallon (mpg) in city driving and maybe 15 to 22 mpg on the highway, but that's a very broad range and those numbers are estimates based on the era. Modern vehicles with the same engine displacement would likely have much better technology, resulting in better MPG, but this isn't the same engine.
To get a more accurate idea of gas mileage, you'd need specifics about the vehicle it's in.
