OK, well over 600 posts to this thread and a quick review didn't find anything that explained my situation (sorry, I didn't look post by post).
My story.
2010 1LT A6.
Took a 1000 mile trip in Feb on mostly interstate toll roads. Average speed of about 67-68mph. Got 30.8 mpg on the DIC. Manual calculating was about the same. I am happy. Mileage was about 10,000 at that time. Combined city/highway, back and forth to work (10 miles each way), I was averaging 20 mpg. Still happy.
A couple of months ago, I installed a Roto-Fab cai and a DynoMax VT axle back exhaust. Been driving back and forth to work and getting about the same, maybe about 1mpg less. It sounds awesome, especially at full throttle, high RPM.
I haven't had a chance to take any long trips, so I just assumed I would get the 30+ or higher mpg.
We left Saturday morning for the in-laws. 125 miles each way. Fresh fill up on the way out of town. Got on the highway, reset the DIC and drove 100miles at exactly 70mph. I had made this same trip before my mods, at 70 mph and gotten 29mpg pretty consistently. Saturday, I was at 23.5 mpg. I was like, WTF? No heavy acceleration, no stop and go driving, just a steady 70mph for those 100 miles. Towards the end of the trip, about the last 25 miles or so, the speed limit is 55 mph. Reset the DIC again and drove exactly 55 mph. Mileage increased to 28.5 mph when I am used to getting 31 on that stretch (before mods).
So I'm sitting up at my in-laws stewing over what is happening. The car seems to have developed a small stumble from time to time. I couldn't pin point anything specific so I just figured it was my imagination. Got to thinking maybe I picked up some bad fuel. My mods couldn't be causing this could they? Hadn't read anything about it here, so I don't think so. I begin to wonder about the ECU maybe picking up some bad habits (probably from my bad driving habits, hehe), and had adjusted the A/F ratio, timing, cam timing, something. So I decided to try to reset it. Disconnected the battery for 30 minutes and reconnected. Took off this morning for the drive back home. Same route, same weather, same everything. Drove the 55mph for the first 25 miles and got about 32mpg on the DIC. Hit the 70mph zone and reset the DIC again, drove the 100 and ended up at 30.5 mpg. A 7mpg gain by disconnecting the battery for 30 minutes. I guess it really shouldn't be called a gain, but more of a recovery. OK, I recovered 7mpg. Spicoli is happy again.
OK gals and guys, anyone ever heard of anything like this?
Thanks in advance for the feedback.