Just as I expected. I just thought it's too irresponsible to advise anyone else to experiment with their vehicles. My own is another question... we don't have any lower grade gas than 95 RON (AFAIK it's similar to your 91) so no room to experiment here, but in May I tried how Teresa likes E85. Well, she hates it. With 50% ethanol she kept stalling and while I could get away with 30% for that single tank it felt so much worse (and the fuel consumption went up too) that I won't try it again without auxiliary electronics... which I won't install anytime soon either.
I don't seem to be getting worse mileage on midgrade. However, in extremely hot temperatures, or if I plan on riding it hard I do put premium in it. Lately with the really cold temps I have been sliding in an occasional tank of 87 in it.
Come on.... Is the 30 bucks a year your gonna save put your kids thru college. They put the octane requirement on the bike for a reason. Do not whine about a re-build if it detonates and you smash a piston. IMO use what they tell ya to use but it's your bike,do what you want
Japanese motorcycles use (R)esearch octane, and we here in the US use R+M/2 According to wilkipedia, 91 R is about the same as 87 R+M/2. I have an older Japanese bike , and have no issues at all with the 87 in a motorcycle that specifies 91 Research octane. But of course you have an Amercian bike, so is would seem that 92/3 is what they intend on running in it, with a little breathing room. Larger cylinders tend to need more octane, if everything else is equal. The 89 plus some 93 thing seems as tho it would be right on as far as octane goes...
harley's usually have higher compression to if i remember correctly which will cause it 'knock' when the fuel pre-ignites from not being the right octane level. this will cause damage and costly repairs. so i'd recommend using the OEM octane recommendations.