Exhaust restrictions can compromise engine performance. Anything that compromises engine performance affects fuel economy, as we all know by now. Many people want to condemn the catalytic converter when they have an exhaust restriction. To some degree, they are correct in naming this the prime suspect, but it’s not the ONLY suspect. You could also have a bad pipe or muffler that’s causing the restriction, so don’t rule those out, especially if you’ve already replaced your catalytic converter and still have an issue.
Exhaust restrictions are actually not that difficult to diagnose. All you need is a vacuum gauge and perhaps a buddy to work the gas pedal to do the test. Here’s a video on the process.