97 dokota v8
2 Answers
It could very possibly cause a loss of fuel economy. The exaust system pressure does contribute to the overall performance of the vehicle. You say "like bad", I'm not sure how drastically the muffler will effect performance, or how bad "like bad" is. But it will have an effect. You could have other issues contributing to the vehicles poor fuel performance, possibly the cat?
it's probably not helping. the 4cyl muffler may have smaller piping, which is restricting airflow.