When I suddenly dropped to about 80 miles for half-tank, it was because I had a fuel leak at the top of the fuel pump. If you don't already know where to access that to check, just remove the bottom portion of your driver side rear seat, unscrew the cover that is under the seat. See if you have a puddle of fuel on top of the pump.

 

It's simple; low compression, low octane ....

 

Thanks - On my second take of 93 - looking about 40 miles to just over 1/8th tank consumed. I'll finish this tank, then swap back to 87.

I don't have a way to scan the OBD2 info, though.

 

You should get one. $15 from amazon, free OBD2 app or knock the total cost up to a whopping $20 for the $5 paid version of torque (android, don't know iphone apps). Read, clear codes, see life sensor streams, take logs, run mode6 tests, etc... on any car you own, or friends, or family, etc.. produced after 1995.