Q: using higher octane on 2005 Honda Civic

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i modified my car in air intake & using iridium spark plugs--currently I am using premium gasoline, I know what is required is 86% octane, What will be the implication of using the premium gas, I also notice that as I use this gas I get more mileage, can you validate my thoughts in this presumption?
(1) Answer
RepairPal has an article with some background on octane:

The best rule of thumb is to go with what the manufacturer recommends. Your modifications will likely have no effect on what octane level is best--it's a question that involves multiple sensors, the engine computer, and the design of the engine overall. However, even the highest performance cars that "require" premium 91 octane will run perfectly on 87. You may lose some higher-end performance with the lower octane gas.