Very true. It takes quite a few fill ups to get an idea of how your car is doing. So many factors to contribute.
"quite a few" is an understatement. I drive a lot. I fill up at least once a week. My commute to work is currently 39.x miles, and before Thanksgiving, we're moving and it will be 47.x miles. Multiplied by 10 trips to and from work each week ...well, you get the idea.
I've tracked over 12,000 miles since February 27, 2015 and gotten anywhere from 29-38+ mpg. My average is 33.8mpg in mixed driving (I say "mixed" because, although i'm technically on the highway 99% of the time, stop-and-go traffic during the drive home is no different than city driving...) But what I was getting at is that
after 12,000 miles, my average is still climbing. In Northeast Ohio, we get a different fuel mixture in the winter months, and we run the AC a lot from June through August when the humidity is 100% (I also wear a suit to work daily, so I'm not going to sit there and sweat...)
Anyway, use fuelly or something similar before freaking out about economy. Fuelly is free. Having the car inspected is expensive. Also check tire pressures. I roll at 35-38. The only 'manual' factor in Fuelly is the percentage of highway versus city driving you do. I also add notes in my fill-ups to remind myself that I had 3 people and 2 coolers full of beer in the car on that 500+ mile trip to Nashville where I "only" got 32 mpg, or that I was driving to Indianapolis and back (no AC on the way there!) for the tank where I netted 38+ mpg.
On my other vehicle, I have separate profiles for everyday driving and towing.