my mustang has been super good on gas until the other day.i see no leaks,smell no gas to indicate a leak.i was getting around 28 mpg.now around 10 to 12.i can go a few miles up the road and notice a drop in the gauge when i return home.it's so dramatic of a change i am flabergasted by this.i have searched everywhere throughout the fuel system for a leak and found everything dry as a bone.i thought maybe spark plugs but it runs smooth and i changed them a few months back.the only thing i've noticed is my catalytic converters have loose baffles inside which makes it sound muffled and power has dropped.could this make it suck fuel.
fuel mileage went from approx 28 mpg to 10 mpg overnight on 1999 Ford Mustang
Check first that the air filter is clean and no rodents have built a nest in the air filter housing. Check the fuel pressure and fuel pressure regulator. Plugging in a scan tool you could read live sensor data to look at air flow into the engine, coolant temperature (if the engine thinks its cold it will richen up the mixture), Oxygen sensor values and fuel trim values.
woa!! you raised a red flag when you said catalytic converter baffles.the converters on your vehicle have a ceramic grid that when go bad,will crack and crumble caising a massive restriction.usually though there is a root cause to their failure,like a ignition concerns,misfire,high fuel pressure.basically they fail when they are overheated or physically damaged.unburned fuel flowing through them will overheat them.did you first replace the spark plugs because it was running rough? if so this may have been the root cause of your cat. converter failure.