There's a station local to me with above ground tanks. They're in an area that flood regularly, which is why the tanks aren't buried.

I've never noticed a measurable difference when filling up the 20 gallon tank in the truck.

I suspect you'd have to buy a LOT to make a meaningful difference.

As I understand it, petroleum companies provide "winter mixes" of fuel that accommodate for the difference in density, so there isn't a real advantage to this.

Summer fuel blends are an emissions control.

https://www.caranddriver.com/news/a15339380/the-vapor-rub-su...

Above-ground tanks can be appreciably cheaper, mostly due to increased inspection and maintenance requirements for underground tanks due to a history of leakage problems. That said, the price difference isn't big enough to justify the use of space (especially considering setback requirements) in urban environments. You will still find above ground tanks in newer gas stations in rural environments, though.

Department of weights and measures calibrate and confirm the amount pumped twice a year. The amount dispensed should be compensated for temperature.