Poor gas mileage after oil change?

[sullybosco]

I'll be brief. Just recorded best mpg ever with me driving, 39.1 mpg. Then, 26.9 out of nowhere. No changes in highway/city mix, driving style, etc.

Things that happened in order of note:
-oil change on 10/5
-cleared old dtcs from several months back
-record 38.2 and 39.1 mpgs, sequentially
-filled up with gas and it stumbled as if i had tried to over fill the tank but i stopped at the first click as always
-yesterday noticed i was at 115 miles on a 1/2 tank. Usually closer to 200.
-cleared oil light
-recorded 26.9mpg

Other notes:
-AFR hovers around 14.7 under constant throttle, around 40 when cruising/shifting (no throttle), drops to 12.4 under heavy throttle
-seemingly no loss of power, boost seems normal for a tune, ~23psi
-oil may be slightly overfilled (can't see the word "max")... But two of my best mpgs ever with it overfilled?
-disassembled and reassembled intake to verify no vacuum leaks
-no smell coming from the brakes take 5
-accidentally pressed one of the cruise control buttons and RPM gauge dropped to 1k for a second...but engine RPM did not change.

What am I missing? Ask me questions. Just a bad tank of gas? Something else?

Is it normal for the MPG to drop after changing oil? Did mine at Walmart and the range went down from 450 miles to 250. I was like “WTF”. Then it slowly started to go back up until 320 or so. But it’s still worse, anyway. Used the same synthetic oil, 5W30. I guess it will become thinner over time and increase MPG? It’s a Passat 2015, diesel. Mpg efficient my ass.

 

[Intuit]

range went down from 450 miles to 250

Calculate mileage manually. I got into the habit of using my cell cam to take a photo of the trip meter (or odometer) then the fuel pump showing gallons.
Look at your manual to get an idea on how range is calculated. Some vehicles will reset if the battery is unplugged or (maybe) a trip meter is reset.