Being that I'm in the Gas biz let me explain this one to you:
I thought you were in th PR business for the DR -
Those plastic cans are not precise! They don't always fit what they say they do and in your case that may be what happened.
Understood that they are not precise.
On the other hand, 5 gallons and 5.5 gallons is a huge difference! There's no way to make this ok!
I also thought that being 10% off was a very large difference. It was 100 pesos more than I expected to pay for 5 US gallons and it was still not quite to the "fill" line of the can.
To test you gas can for calibration when it's fully empty, use a gallon container and fill it up with fresh tap water. Then take the gallon of water and using a regular home/bath scale check that the weight is within 8.34 to 8.35 Lbs, which is the US gallon limit weigh for water.
Mark the gallon with a magic market to the fill line, empty the gallon you just used on the sink and continue to refill the container this time using the water to fill the 5 gallon can with it. Don't pause between the fill up from the tap water as temperature will change in the can rapidly!
When all the 5 gallons of water are poured in the gas can, mark the fill line and note the temperature outside the home.
The next time you go to the gas station and pump the gas can, make sure to have them pump exactly 5 gallons on the can, using the temperature recorded on it, make sure it doesn't go beyond the 10 degrees in that level (yes 10 are still ok with gas).
take the can home and place it a cool spot away from the sun for a while until the ambient temperature matches what was recorded on the can.
The fill line on the can you made with the marker must match the actual gas in the can as well. It could be off by mere millimeters at worst only.
If you find a huge difference between the lines the gas station's pumps need calibration or they're aware of it.
Good idea, but I would trust the scale I have less than the can, so I won't be able to perform such an experiment without injecting another variable into such an experiment.
The pump will always give you the exact gallon of gas no matter the expansion/temp or anything else for that matter. That's a given!
No question that the gallons pumped should be exactly as requested regardless of the temperature of the gas. The volume of the gas will only change later if the temperature changes