[rdl]

Usually additives are added to premium which wouldn't be in mid-grade (not a huge cost difference, though). Usually this stuff gets mixed at the rack (along with ethanol, etc.), although there are older gas stations and cheaper stations which mix high and low on-site at the pump for medium.

[ars]

> Usually additives are added to premium which wouldn't be in mid-grade

That's the perception (and they are happy for you to think that), but I don't believe it's actually true.

And in any case, if it is true, a mix would simply have a lower level of additives for midgrade and that seems right to me.

> although there are older gas stations and cheaper stations which mix high and low on-site at the pump for medium.

I'm almost 100% certain that almost all gas stations have just two storage tanks and the pump mixes them.

See: https://blog.gasbuddy.com/posts/Gasoline-pumping-from-the-st...

http://hyperbear.blogspot.com/2006/05/dirty-little-secret-ab...

[rdl]

Ah, it's apparently 90% have two tanks. The remaining 10% have 3 and have pre-mixed, because they had leftover tank from when they did leaded gas, and by doing this they don't need to upgrade the pumps. So it's probably old/cheap/un-upgraded stations which have 3 tanks.