I've noticed that over the past two fill-ups I am getting only about 300 miles per tank full vs. my previous average of 350 miles. No change in driving style or gas station. Due for my 45K service soon, anything I should have the dealer look at?

That was the first indication that my TC was going bad....and then the mileage went down to 285 then 250 miles........I started filling the car up three times a week since the local mileage suffered even more. Check to see if your RPM's are 3000 at 60mph then you can assume the tranny is going bad....to save your sanity fill up with 89 octane otherwise this becomes expensive.....real fast

Thought it was just coincidental to mounting up the snows which will have greater rolling resistance than the Toyos. Glad to know I'm not the only one. But 13.5 mpg in the same basic driving I used to get 16.5-17 is a huge drop.

IIRC this includes plugs, filters, etc. Perhaps related. Perhaps the others are right that your Tranny is slipping, or the gas is a different blend.

for a tank or two before I have had a coil pack go bad. usually the bad coil caused a drop worse than that though.

Also your OXS sensors might be getting old which usually manifests itself as a drop in the gas mileage. If you have a Lambda meter you can observe a gradual decrease in the oscillation frequency as the OXS sensor ages.

When should it be changed?....I noticed a decrease...

I realize that it is high but that was the way I knew I had a Torque Converter problem after the tranny was rebuilt......I hope it comes back with this issue resolved

MN runs 10% ethanol year round and still see a decrease in mpg with the winter mixture. I also find that if the average speed on the computer is around 30 mph vs 40 the mpg is worse. Indication of more stop and go traffic.

In older Audis I use a 3-wire generic Bosch which costs ca. 30 bux. I wonder if anyone has come up with a generic 4-wire sensor p/n yet?