if i were you i would get a retune. but the elevation part is right, but if u were tuned on 93 then u need a retune for 91.

here in colorado we ONLY have 91. there is no 93. its something to do with less air mass because of the low density of the atmosphere at high altitude. so less fuel and reduced absolute compression in the cylinder, therefore deterring knock.
(partly taken from wiki)

 

supposedly 20% mixture of toluene will increase 2.5 RON. ie up the rating by 2.5.


based on tests some place did.
perform at your own risk. as toluene is a special order product.

 

here in cali, there is no 93 the best we get is 91. I would think that you would be okay driving on 91 just don't stay in boost for prolonged periods of time, then again if the car was tuned for 93, then i'd recommend a re-tune. If you have no other choice, and you are forced to use the 91, granny shift and stay out of boost til the re-tune.

 

I believe around 10% will bump it to 94-95 octane

 

yea here in cali we never have 93. we only have 91 : /

 

may be a dumb suggestion, but cant you just throw in some AMSOIL octane booster in there and be good?