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.
Pretty much that, the higher the octane rating the higher the temperature before pre-ignition (Knocking/Pinking) will occur. Most modern cars ECU's will continually advance ignition timing to an optimum level until knock sensors pick up any knocking occurring then back it off a bit, so cheaper lower octane fuel will allow knocking earlier ergo, timing retarded earlier resulting in lower performance and slightly higher consumption