My FE is falling and I don't know why

[theholycow]

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...