Common problems that decrease fuel efficiency?
 

Ok, so I've noticed my 2000 A4 2.8L 30V seems to be guzzling gas more than it did. Driving about as gingerly as I can around town I'm getting 15mpg at best, and I'm in a small town with very little waiting at lights. On the freeway doing 75mph on cruise, I'm getting about 23-24mpg. It just seems like I should be getting a bit more than that.

I get the following codes when a VAG my car:

3 Faults Found:
18014 - Rough Road/Engine Torque Signal from ABS: Electrical Malfunction
P1606 - 35-00 - -
17831 - Secondary Air Injection System: Bank 1: Insufficient Flow
P1423 - 35-10 - - - Intermittent
17819 - Secondary Air Injection System: Bank 2: Insufficient Flow
P1411 - 35-10 - - - Intermittent
Readiness: 0000 0000

Not sure if any of these my affect economy. I just cleaned my MAF/replaced air filter, and I have yet to really test that, but any other ideas? I know one guy said he figured out his purge valve needed replacing when his economy went down.

make sure that your tires are inflated properly, you can believe how much that matters.

also remove all the dead bodies from your trunk you wont believe how much weight it adds to your car. Remove all the bugs that got run over by your bumper on the highway. Only add 50% of the windshield fluid in the tank. Empty your glove compartment and your ash tray. You should see a great increase in your MPG performance. Good Luck

Hahaha!
(i know, uninformative post, but it was funny!)

The ABS rough terrain code is pretty uncommon from my experience. Pin test the data bus at the ABS control module to the data bus on the ECM with an ohm meter. There are specs for the resistance, but not in front of me. Do you have a friend with Alldata?

The two SAI codes are probably affecting your mileage. Vacuum leaks will generally cause the out of spec codes for the SAI since the control solenoids regulate vac pressure, check those too. It could also be your pump is breaking at the rivets or the air hoses are cracked and broken. Grab a flashlight and poke around a bit.

Work on fixing those codes first, since that is what the car thinks is wrong.

+1 on fixing the codes first, that's always the best place to start.

After that, try running some seafoam through the fuel system and - if you're due for an oil change - the whole engine as well. Gunked up injectors and sticky lifters can cause all kinds of inefficiencies.