New cat/02 sensor/muffler & I LOSE 3 mpg?

This is enough to constipate a goose. I just had a new cat/front 02 sensor/muffler installed. They look gorgeous but instead of the increase in gas mileage I was anticipating, on the two 2 fillups since this work I'm getting barely over 15 mpg instead of the 18ish mpg I got on my first 3 or 4 tanks.

Any possible explanations for this or little tweaks I can try? Plugs/wires/cap/rotor/air filter/fuel filter all new, no difference in where/how I've been driving, very little change in AC use. I'm flummoxed.

Unplug the battery and let it sit for a while so the computer has to relearn the parameters. It will take a while driving around till the comp gets back up to speed. It might help. What brand O2 sensor was used ?? And was it the first sensor, the one on the downtube or the one on that cat ??

I'd heard (from that insufferable 25 mpg-ZmOz) about disconnecting the battery, but Eagle--if I understood him-- said that wasn't necessary since the comp would adjust in 5 off/restart cycles. But I'll do it anyway, of course; can't hurt.

What I asked for (and assume was put in) is the "front" 02 sensor per Eagle's suggestion. You suppose my mechanic replaced the wrong sensor? But surely that wouldn't make my mileage worse than it was when I had a rattling cat--would it? Both the cat & sensor--& muffler--are Warner, I believe.

Rich, as you prob. guessed they are not Warner but Walker. (The first 02 sensor that was priced to me was a Borg-Warner that wouldn't do, and I think he ended up ordering all three parts from Walker.)

Brain cells are getting on up there.