You have it backwards. At higher elevations there is less pressure, therefore the engine compression is naturally reduced and lower octane is needed. That's why gas stations in places like the Colorado Rockies offer as low as 85 and not much higher than 89.

At lower elevations there is more pressure and more compression, which requires higher octane.

 

I agree completely. Not only that, but I would hate to be in the position of not being able to afford 91.

 

Where I am, we only have the option to buy 87, 89, or 93. It does kind of annoy me to have to put in 93 when 91 is required, but what can you do. Around here there's barely any difference price-wise between 89 and 93.

Maybe when the tank gets 3/5 full I should top off with 89?

Here's a question - when you put in a higher octane gas than required, does that hamper performance at all?

 

^^^

It's 91 or higher ocatane

 

^^ what he said. It won't hurt, but also won't help.

Like you said, there's barely any price difference with the 89 and 91 so it's not worth tinkering to me, but I certainly don't fault the OP for trying.