Harley recommends 91 octane fuel, but since I'm not running the bike hard (I'm very rarely using more than 25% throttle), and it is computer controlled fuel injection, what are everyone's thoughts on me switching to 89 octane fuel? Saving $0.15/gal would be nice.
My Honda CBF has got a lot of sensors (throttle position, MAPsensor, coolanttemp, airtemp, lambdasensor, bank angle sensor, rpm...) but I've never seen a motorcycle with a knocksensor.
My opinion: don't change to lower octane rate as manufacturers recommandation. You'll see that on that hot day, with a hot engine you need some power (think about your safety) and your engine will not deliver as designed and there's a risk on engine damage.
I would only risk it with a knocksensor, but even then your engine might not perform as expected.
In my area there's generally a $0.30 - $0.45/gal difference between 87 and 93 octane. You are right though, it is only a 5 gallon tank, and I haven't let it get under 1/2 tank so far.
I think I can do a good compromise. Since 91 is recommended, and stations in my area carry 87, 89, and 93, and I've been filling up at around 1/2 tank if I alternate between 89 and 93 then I'll end up with 91 in the tank.
motorcycle manufacturers have to factor in all types of people riding in all types of conditions. big people, small people, riding with a passenger, outside temperature changes,altitude, etc. If you are not riding hard, and no passenger, mainly commuting to work, I would try a lower octane fuel. If it pings, drop a gear, and let off the throttle,it's not rocket science.