Not going to address alcohol content? You didn't try some of that cheaper E85?

 

Obvious question to me anyways... How many miles are on the car?

 

Injectors sometimes begin to stick. Have you pulled any codes?
Something may have gone south, should throw a code I would think.

 

Cat or O2 sensor could cause a decrease in mileage.

 

That is exactly where I was going. I have seen situation where the O2 sensors are lazy and the fuel economy goes down, replace the sensors fuel economy is restored. FYI the if the front sensor, that is the one that has an impact on the trims, the only reason for the 2nd is to check cat efficiency.

The mile mark where i replace them is around 100k. In the situations where I've seen it impact economy the mileage was in excess of 100k so that is what I use for my reasoning.

 

Could be, but:
"lazy" is measured in milliseconds; if lean/rich transition is more than 200 milliseconds or rich/lean transition is more than 130 milliseconds there would be a P0133 code.

If the cat was plugged up there would be a P0420 and other symptoms, like greatly reduced power.

 

Indeed. The Cat on my fathers avalanche broke up and clogged itself. The truck wouldn't rev past 1500 RPM, had random misfiring, spuddering, terrible gas mileage. fixed the cat and it was like brand new. They can give some weird symptoms. Lead you on a wild goose chase.

 

Add some lucas or techron to a tank

 

If the cats core has been contaminated with either oil or Dexcool , additives area going to clean it out.
I cut open the cat on my 1988 after it had a melt down and the core was melted solid.
That caused my brand new 355 SBC to have a bad reaction and crack the block and passengers side cylinder head from the pressure, and me putting in additives, thinking it was going to clear it up.
Another new 355 and new hiflow Magnaflow cats, cured the problems

Cha-Ching!! $7,500.00 in damages.

 

Well I got home today and now my mileage is down to 24.5 highway. I replaced the downstream o2 sensor because that's all anyone in town had. My car has 145,000 miles on it. I still am not getting any codes. I have to leave for Oregon tomorrow due to a death in the family so I'll see what happens. It's 5,000 miles round trip so maybe it will set a code so I can see what's going on. I may be able to find an upstream 02 sensor out there.