Seen improvement with higher octane also, but it was a turbo charged engine designed for higher octane. GM just labeled it regular gas for marketing. The fuel economy improvement wasn't worth the price increase, unless I mixed my own midgrade, but that wasn't worth the hassle.
Without the higher effective compression ratios, you won't be able to take advantage of the higher octane. That said, there are other factors in play that could explain fuel economy improvement in using premium in a low compression engine. My first thought is that premium gas tends to have better detergents added. Cleaning out the fuel system will improve things.
Then cars advance ignition until knocking is detected, and then back off slightly. This cycle is constantly repeated. With octane higher than what the engine can fully use, ignition timing advances as far as possible. This could mean better fuel economy and performance as the max point for the car is likely beyond what it runs at with regular. Then the engine should run smoother as it isn't always on the cusp of knocking noticable to you.
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