Does using high octane gas (91) give better mileage for a car?
You notice that all of the responders saying higher octane fuel gives more power haven’t posted dyno numbers for their comparison. Funny how your “butt dyno” can be deceived by misperceptions. Higher octane fuel takes higher temperatures to burn completely. The higher the octane, the less combustible the fuel, and consequently, will make slightly less power in an engine not designed for that fuel. Therefore, in your civic, you will notice a DROP in MPG by switching to 91 octane. Higher octane fuel is more resistant to predetonation (a phenomenon experienced in high compression engines). The ideal gas law (given by the formula pV=nRT) describes how pressure and temperature are directly proportional. As you increase pressure, temperature will also increase. Your lower octane fuel/air mixture can ignite without a spark if the pressure spikes in a high compression engine (high load conditions – going up hill, dropping the clutch at a red light, etc) due to the sudden increase in combustion
