"No good" doesn't say anything. What is wrong with these parts? If the muffler is rusted out already a baffle could be collapsed and blocking exhaust gas flow. That can make the engine work harder to push the exhaust out. If something was crushed, that will also restrict flow.
A leak before the front oxygen sensor can allow outside air to be drawn in between the pulses of exhaust flow. The oxygen in that air can be detected by the oxygen sensor as a lean condition. The Engine Computer will respond by commanding more fuel to enter the engine.
Both of those conditions are not real common, and it's impossible to say what the impact on fuel mileage would be without knowing more details.