ek49-ret

I have an '87 Pioneer with the 4.0L 6 engine; I recently changed the exhaust man. with a new stock model w/o the side tube. My mileage has drastically dropped from the standard 15-20 mpg to 6. I changed nothing else, and since this was a relatively simple swap-out, I'm puzzled. Does the external tube matter, or if not, what do I need to troubleshoot?