* Improper installation: A poorly installed tank could lead to leaks (which would be obvious) or affect the fuel system's operation in subtle ways. Perhaps a sensor wasn't properly reconnected or the fuel lines are kinked, restricting fuel flow.
* Damaged fuel pump: During the tank replacement, the fuel pump (located inside the tank) could have been damaged or dislodged. A failing fuel pump can't deliver fuel efficiently, leading to poor mileage.
* Faulty fuel level sensor: If the fuel level sensor was disconnected or improperly reconnected during the replacement, the car's computer might be receiving inaccurate information, affecting fuel injection timing (though this is less likely to cause a major mileage drop).
* Other unrelated issues: The tank replacement might be coincidental. The car could have developed a separate problem around the same time—a failing oxygen sensor, a clogged air filter, spark plug issues, etc.—that's actually the culprit behind the worse gas mileage.
* Different tank: While rare, it's possible the replacement tank is not an exact match for the original. While unlikely, this could theoretically affect fuel flow and pressure.
In short, the gas tank replacement itself is unlikely the direct cause; something related to the installation or another concurrent mechanical problem is more probable. If the mileage decreased significantly after the tank swap, it's crucial to have a mechanic investigate the fuel system to pinpoint the real reason.
