sudden gas mileage drop

I've got a 98.5 A4Q 2.8 car with 95k miles on it. It runs great, smooth, more-or-less quietly, and with normal acceleration, but in the last 3 tanks of gas, its mileage has dropped from its usual 22-24 mpg
down to 18-19 mpg. What should I be looking for as the culprit?
This mileage drop started during warm weather (40s), so I can't blame it on cold temps, and my tires are inflated properly. The car is a MT, also, but I'm the only driver for the most part. Suggestions?
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Is your water temperature right? A bad T'stat will cause low running temps will cause the ECU to overfuel the car.
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Water temp seems fine, always reads at the noon hash mark, sometimes wanders up to the next hash mark (the pattern for this particular T-stat, which was put in at 69k miles during the TB/water pump changeout). No change in behavior there. Should I get this tested anyway?
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KLS, I'd check the mass airflow sensor and the oxygen sensor(s). You can do so without removing them using an voltmeter that has a "duty cycle" setting. I'm sure you could get the specs from the A4 newsgroup on Audiworld. Cheers! Steve Sears 1987 Audi 5kTQ 1980 Audi 5k 1962 and '64 Auto Union DKW Junior deLuxes (SPAM Blocker NOTE: Remove SHOES to reply)

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Never seen a correctly working stat cause the temp to wander above the mid mark. That could just be a faulty temp sensor (not uncommon on VAG engines) & that would screw up the ECU as well. I think its a bit of a fiddle to change on the V6 but DIY-able. Have a look at the Audi forums for a "how-to". Shouldn't be too expensive.
If the MAF or the oxygen sensor was bad I'd have thought the thing would be running rough but they are worth checking as well. The temp sensor will be a lot cheaper than a new MAF (not sure what the O2 sensors cost)
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US fuel formulation changes (with the addition of MTBE) for the colder months resulting in a somewhat lower mileage. Colder is all relative depending upon where you are and when area refineries decide to switch over. But winter fuel would only change your mileage around 2mpg.
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You are totally right, and I have known this for many years, but this is the first time I've experienced such a serious gas mileage drop, and I've owned the car now for almost 5 years. The car and I have lived together in the same cold western New York region, so believe me, we have gone through the normal winter mileage drop, but nothing else has changed in our environment that would account for this weird drop.
I'm going to try Steve Sears' suggestion to check the MAF and O2 sensors; I'd been thinking about those as possible culprits. Thanks!
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