if you buy it at the pump, it's low sulfur.  the point is, there's a
difference between fuels, and mileage numbers can be easily fudged by
supplying fuel with a high energy content for official tests, while the
stuff you and i can buy on the street doesn't come up to the same
standards, hence the difference you're seeing.

concerned?  you should be!  lowered energy content fuel is a scam
because fuel is sold by volume.  when you buy natural gas that's piped
into your home, it's regulated by law and sold by energy content not
volume.  interestingly, gasoline and diesel are not so metered.  that
gives the oilcos considerable scope to "volumize" their fuels, exactly
like a bartender watering down spirits.  lower energy content = lower
mpg's = buy more fuel to do the same journey.

considerable resources are devoted to ensuring gasoline pumps don't
short-change customers, but amazingly, all this regulation avoids the
elephant in the room, that of energy content.  ALL fuels need to be sold
by the calorie/therm/btu/megajoule, /not/ by volume.