Costco Gas competitors

Who would compete in quality and price for Gas with that of Costco?

My spouse made a observation that the mileage you get is low if you go to a cheap gas place.

Any other places to buy gas in Northern California that would compete in price and quality with that of Costco?

This is in CT but both my wife and I noticed getting less mileage when we fill up at “Freedom Fuel”, a place that gets their gas from Venezuela, or so I hear. The gas is often 15 cents cheaper there so I guess you get what you pay for.

All gasoline sold is made to standardized quality levels. The energy content of gasoline is the same no matter where you buy it…Driving conditions cause variations in fuel mileage, not the brand of gasoline you use…

I know it is supposed to be but that Freedom Fuel does not seem to take me quite as far as others - anecdotal, no doubt, but it just seems to need to fill up more frequently. The commute is pretty much the same and I drive pretty much the same, around 65.
I’ve been curious about the actual numbers so am tempted to document it now.

If all gas is indeed the same, I’ll go where it is cheapest. No point rewarding greed.