My mileage dropped of on my '90 Sentra awhile back. It was getting 33mpg mixed and dropped to 30mpg and then to 26 or so. I had a new Air-flow meter, new plugs, new O2, etc. so it wasn't any of those. As a last resort, I cut my catalytic convertor off, installed a straight pipe, and instantly gained my mileage back.
I have a new one along with a new resonator and muffler to go on, so no tree-hugger comments
