I used Arco for years without issue, same as any other non-top-tier (by "top-tier" what is meant is name brand, Texaco/Chevron, Conoco, Shell etc) gas.

Octane is different. You need to use recommended octane for your car but not more.

In your case, it's likely that (probably a turbo, otherwise something higher-compression-ratio) the car is compensating by retarding timing due to sensing knock when you use lower octane. So I expect a) premium is recommended and b) that's why you are seeing an efficiency (and performance, you may notice) gain.

Note, this octane thing is why mountain states still have premium at 91 instead of dropping it like the other grades - because turbo motors compensate enough for the altitude to still require it. With the advent of lots more turbos (thanks Obama) some (like Honda) recommend/work with 87, so that in most places regular gas works for them - but in mountain states this means you still should follow the recommended octane vs the corresponding grade.

Edit: I have no idea why you had poor results with Arco. That's an interesting data point but usually it's psychological - perceived performance - not objectively measurable like mileage. I wonder if they cheat on octane rating (my experience at the time was car that required premium so perhaps better there)? That's all I can think of....

I used Arco for years without issue, same as any other non-top-tier gas.

Octane is different. You need to use recommended octane for your car but not more.

In your case, it's likely that (probably a turbo, otherwise something higher-compression-ratio) the car is compensating by retarding timing due to sensing knock when you use lower octane. So I expect a) premium is recommended and b) that's why you are seeing an efficiency (and performance, you may notice) gain.

Note, this octane thing is why mountain states still have premium at 91 instead of dropping it like the other grades - because turbo motors compensate enough for the altitude to still require it. With the advent of lots more turbos (thanks Obama) some (like Honda) recommend/work with 87, so that in most places regular gas works for them - but in mountain states this means you still should follow the recommended octane vs the corresponding grade.

Edit: I have no idea why you had poor results with Arco. That's an interesting data point but usually it's psychological - perceived performance - not objectively measurable like mileage. I wonder if they cheat on octane rating (my experience at the time was car that required premium so perhaps better there)? That's all I can think of....