Here are some observations I have made in the past month as far a fuel economy.
I live in Ontario Canada we have four seasons here, being fall it is cooling down,
1)cooler weather means poorer fuel economy.
So my MPG have dropped due to the air temp.
2) summer holidays are over, school is back in and there is notably more traffic on the roads to and from work. this heavier traffic has lowered my MPG.
3) there is more construction going on right now on my rout to work. this has cause slowdowns and has effected my MPG.
Some or all of these factors should be taken into consideration when doing your mpg calculations.
There are sooo many variables involved it sometimes can drive you nuts!
Also very important!!!!
You must always fill up at the same pump in the same exact spot each time you fill up, if you want an accurate record of your MPG. different pump shut off at different tank fill levels. I found this out the hard way.
Mother Nature educates all of us that are teachable. She's hardest on the ones who refuse to learn. Punishment is automatic, immediate, and without pity.