My FE is falling and I don't know why
 

My FE (for Effram) is falling and I don't know why. My first two EOC-heavy tanks were >42mpg. The next two were ~38mpg. I got dependably over 35 when I perfected engine-on P&G.

The only possible causes I can think of are weather and traffic. The first >42mpg EOC tank was on 6/20 and was during hot dry weather and good traffic conditions, but the second >42mpg tank on 7/16 was during sub-optimal weather and traffic similar to the two more recent lower tanks.

Any suggestions as to what could have caused it?

You can see the two peaks at 42mpg...
https://www.gassavers.org/gaslog/chart.php?id=1399

That looks to me like a steadily rising average. It is very normal for the tank-to-tank measurement to bounce around. You need more data before complaining.

Of course, I complain when it drops for 1 tank. ;)

I was concerned because previously, my results have always been consistent and predictable, but now I've had two tanks in a row that were far lower than I expected...and almost as low as if I didn't make all the effort for EOC.

Ethanol?

Yeah, my first suspicion was to blame on E#, assuming everything else is the same (engine quality, tires, aero drag, weight, driving)

Another good idea, but probably not it. I'm getting the same gas (even the same pump) as I have been since I got this car, and it's been labeled as having 10% ethanol the whole time.

Everything I can think of, and everything mentioned in this thread so far, has not changed. :(

Perhaps the station is not getting the same quality of gas.

perhaps it IS your driving style, you said you perfected EOC on one tank, and did engine on P&G on another.... also are the route you're driving consistent or have you taken a few side trips? how about the cars condition: when was the last time it was aligned (yes even new cars need regular alignments for those who wonder) hows tire wear and inflation? how are your wheel bearings, brakes dragging, etc.

Have you been able to use EOC as much on the tanks with the lower numbers? Do you know how many actual miles you used EOC on each tank? Just a few miles more or less would make a big difference. It would probably be a good idea to try to keep a mental record of the miles EOC each tank. I'm not saying you have to know right down to the last tenth of a mile which would be nice, but a general idea should tell you what you are really doing. Since you are getting average of 35 mpg if you coast 35 miles on one tank and only 17.5 on the next that's going to be an extra 1/2 gallon burned even if you drive the same distance on both tanks. I just done some figuring and over the last 10 fillups your average fill up comes to 6.5831 gallons so if you use an extra 1/2 gallon on one fill up your fuel efficiency is dropping by almost 7.6% for that tank. So if you got 42 mpg on one tank then drove the same distance on the next tank but only coasted 17.5 miles instead of 35 like on the previous tank that's going to drop you mileage to 38.8 mpg. Just a theroy. Only you know whether you used EOC as much or not.

Your average is still trending up, and you are still doing a LOT better (per your gaslog) than you were just a few months ago. I don't think I'd be too worried...yet.