i believe my fuel door says 'premium fuel only' the highest we have here is 91. buy whatever is 'premium' in your area
before my base 94 toyota pickup got stolen all i put was 91 in it. there was a thread on NorCalDrift (which i believe is no longer around), where a guy logged his fuel MPGs between 87/89/91. his ae86 actually was cheapest with 91. his reasoning was that the car doesn't use as much 91, combine that with feather footing and he was able to save a little money between fill ups that add to a substantial amount in the long run. it went on to say that not all cars are like this and you will have to experiment with your own vehicle.
with that being said (and off topic) your driving style is the biggest factor in fuel consumption, IMO.
Lucy; 04 manual sportdesign