Gas milage

Any advice would be great. I have a 98 tacker automatic. It has EGR problems; I have cleaned and checked the EGR It works on the bench. I will try and clean the system again. It will code after about 500 miles. The main problem is gas mileage is only 20MPG the five speed it replace was around 34-37. The auto is it a four speed or a three speed? It hits 4th at 46 MPH and will drop down a gear under 45MPH. The exhaust is very loud not a leak but a deep rattle. It is in real good shape on the outside I have banded or removed the shielding and am not convinced it is the shielding that is making this noise; the noise is not Tinny enough. Is it a double wall pipe or is my converter plugged? I have had it up to about 85 MPH. My five speed would do 92 MPH.