My FE is suffering

[Jasen]

13-14 years ago I run 3 consecutive tanks of all major brands of gas.
85 Toyota p/u long box 2wd, 2.4l EFI 140kish miles, 12 gal fuel tank, no mods.

As I recall the results

Best to worst mpg mixed driving.

Chevron/Texico avg. 28 mpg

76/Shell 26/27 mpg

Exxon/Mobile 25/26 mpg

Arco 22/23 mpg

So in my experience, there is, at least then, a notable difference in brands.
But a .40+ cent difference off set's 1 mpg loss

[RedDevil]

Though my per tank average is reasonably stable, my worst commutes may use over 50% more fuel than my best (over that tank). Wind and traffic jams are the main causes for that.
My tank average can be 4.0 l/100 km, there might be both 3.3 l/100 km and 5.0 l/100 km commutes in that.
Sometimes those fluctuations carry through on the tank average. If I'd fill up halfway the fluctuations on tank average would be far bigger.

[Diesel_Dave]

Rule of thumb (based on my own data and other Ecomodders' data) is 1% FE drop for every 2 deg F temperature drop. So 4.4% lower and 10 deg F cooler is very reasonable.

Also, don't discount fill variations. Getting to within +/- 5% on just fill variation is tricky.

I personally don't put much stock in fuel brands, but if you want to simultaneously compare cost & FE, the best way to do it is $/mile.

[MetroMPG]

With ambient temps dropping, have you considered grille block(s) and a coolant heater?

[Jasen]

Gas is gas. Shell put's 100,000 gals. in the co-op pipeline in Texas and can take 100,000 out at the other end right now. It's the additives and storage tank condition's that make the difference IMO
The motor bay in my van get's much hotter then any rig I've ever owned, you can slow cook a pork butt in there, if there was room for it.
But I am thinking a partial grill block, I'm stuck in an apartment complex, their not going to let me string 150' cord for block heater.

[XYZ]

A drop in ambient temperature (resulting in slower warm-ups) plus being stuck in traffic would account for your drop in MPG.

You probably can't do anything about either. That's life. Roll with the punches.

[Jasen]

98% of my driving is rush hr. the traffic flow fluctuates from day to day, all my fuel logs are for pretty much the same driving. It's not cold here, yet, low 50's hi 70's 3-4 weeks ago hi was 80-90+