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The lower the octane of the fuel the more of said fuel the car will use.
Citation needed. Generally speaking lower octane fuels have a higher calorific value, but are more inclined to detonation. Unless your car's ECU has knock detection, and backs off the ignition timing significantly with lower octane fuel, it won't affect consumption rates. However, it may well up limiting maximum boost in an engine with knock detection and turbos.
Brommers
@Dammit
The lower the octane of the fuel the more of said fuel the car will use. Whether the higher efficiency balances the higher cost of a better fuel is something I'm doubtful of, but it's still a thing.
Related note - I'd hate to see what my C55 did in terms of MPG on basic unleaded, but as it's forbidden in the user manual I'm unlikely to try it.