From my personal experience:
When we were living in Ottawa, we had 5%-10% Ethanol fulel available form a fairly big chain that has their own refinning facilities. (Sunnoco) I found about a 10% to 15% decrease in my fuel consumption. The improvements were more pronounced in city driving, less so in highway driving.
When we moved to Calgary, which is a 3,500 ft above sea level, and so where the partial pressure of oxygen is lower, I expected even greater benefits, as ethanol is an oxydiser, which means it contributes oxygen to the combustion reaction. However, I did not see any improvement here in my fuel efficiency compared to name-brand regular unleaded.
Now the only chain that carries here it is an independant chain, (Mohawk) and as far as I know, they don’t do their own refining, but rather buy in bulk from other major oil companies. I mention this because I used to work in a refinery as a summer student, in another city, and it was common practice to have a reserve of gasoline intended four our own branded gas stations, and another stock for the independants and wholesalers. Officially, the reason was that we included special additives in our own stuff that cleaned injectors and prevented gas-line water freeze-ups in the winter, but I know that from time to time, there was a fair bit of junked heavy oil products that were just disolved into the non-branded gas. I would burn ok, but dirtier, less efficiently, and would gum up your injectors eventually. It was a handy way of disposing of the stuff for the company, though, and make it’s own gas perform comparatively better.
So basically, I can’t help but wonder if that’s what’s happening here. Because I can’t see how Ethanol would not have an even more dramatic effect at higher altitude. Any chemists, feel free to educate me.