Using 91-octane premium fuel instead of 87-octane regular fuel will only increase your engine's power if your car's manufacturer specifically designed the engine to require it.
Higher octane fuel resists knocking or 'pinging' under compression, which is a premature ignition of the fuel-air mixture. Engines designed for higher octane can take advantage of this resistance to operate at higher compression ratios or with more advanced timing, which can lead to increased horsepower and efficiency. However, if your vehicle's owner's manual specifies regular 87-octane fuel, using premium 91-octane will likely offer no discernible performance benefit and is generally not recommended. Always consult your vehicle's manual for the correct fuel grade.
Fuel octane requirements are determined by the vehicle manufacturer and using a higher octane than recommended typically provides no benefit.
What octane level is best for maximizing fuel economy in my car?
The octane level that is best for maximizing fuel economy in your car is the one recommended by your vehicle's manufacturer. For most cars, this is 87 octane (regular unleaded). Using a higher octane fuel than recommended will not improve fuel economy and will only increase your expenses. If your car's engine is designed with a higher compression ratio or advanced timing, it might require premium (91 or 93 octane) to prevent knocking and achieve its designed fuel economy. Always refer to your owner's manual for the specific octane requirement for your vehicle.