I put on headman headers on my 81 cj5 with a 350.
When I put on CATS - they were removed before I bought the jeep my MPH per gallon went up a few miles.
I agree with the other back pressure has allot to do with it.
What is weird when I put on the air pump my mileage went up more and my exhaust note got louder and sounded allot better.
I feel like I got better low to mid power. That is what I need for off-roading so I really do not understand why people remove emissions stuff.
Hell the EGR keeps the engine cooler. I got proof of that.
After about 2 hours on the trails I am getting 245 with the EGR and headers I am hitting 220.