The only way to really monitor your gasoline usage is to record the miles and gallons for each tank. Get a receipt for every tank, note the odometer reading on it. Put the numbers into Excel or something similar and make a chart. You can see your mileage very clearly.
Be careful, though, it is easy to fill your tank unevenly. Different nozzles can cut off differently from each other for example. Getting the tank filled to exactly the same level each time is not possible. So, when you view your chart, you'll see a zig-zag line. You have to draw a mental line through the highs and lows to get a real picture.
I've been keeping track of my mileage for decades. You can tell by looking at the charts when I've gone on vacation, because the MPG is much higher on long trips than on local trips.
Gas at most stations is indistinguishable from gas at other stations. The only difference between brands is the small amount of additives the different brands add to make thier product "special."
If a station added something to decrease your MPG, customers would soon notice and quit going there. The exception is E85. You can expect about 15% less MPG with E85 than with ordinary gas, ethanol has less heat content per unit volume than gasoline. Diesel, on the other hand, has more heat content, that's one of the reasons diesels get a bit better MPG than similar gas engines.
Cheers, Steve SSOA F00-1363 CCSD #204
2000 Camaro SS Lt. Pewter, neutral leather, M6, Hurst, PEG2, ASR, defogger
SLP Chrome ZR1's, D-D exhaust, Bilstein, Castrol, cover, mats, fobs
BMR STB, SLP !CAGS, AAM Diff cover, Chrome Hood Insert, Zaino
Dyno'd at 312 RWHP, 321 RWT