First I need to say my '72 F100 with a 428 cobra jet only gets about 4mpg. It's the reason I purchased the bike for my commute. With that said I noticed the speedo error. If you haven't changed sprockets your indicated mileage will be wrong. I'm running a 17 tooth counter shaft sprocket a PCIII/USB and get right at 47mpg on 93 octane. I have not gotten a good check of my speedo yet but I have driven with some pace cars and it seems close to right
It's not about $'s I am running the 93 octane because of the increased engine load with the higher grears (low speed throttle). The gear change makes going to second a real kick when you want on the highway. Makes those lane changes more fun.
High dollar, high performance vehicles often require high octane. So people often assume that high octane is "better" because "better" vehicles require it.
Now some people claim that premium fuel is "cleaner" than regular and has less impurities. I've never seen anything factual one way or the other on that. I've seen people that claim to work in refineries that claim the premium and regular come from the same tank and they just do some extra steps to raise the octane on premium.
It used to be years ago that Chevron only put Techron (their excellent FI cleaner) in the premium gas so I would occasionally do a tank of premium for the FI cleaner but now all grades have techron.