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I’ve been using 91 octane in a '99 Mitsubishi Gallant and with todays gas prices, would it be OK to change to a lower octane to save money or would it do any damage to the engine?

What does your owners manual say to use? It probably says to use 87 or higher. Follow the manual.

The folks who designed and built your car are the ultimate authority on what octane is needed by your engine. As was said, the Owner’s Manual will contain this information, and it may also be noted on either the gas filler cap or the inside of the gas filler door.

Incidentally, if the mfr specifies that regular 87 octane gas is correct for this engine, then you have been wasting money every time that you filled the tank with 91 octane gas.

Using gas of an octane higher than necessary provides absolutely no advantage.
Using gas of too low an octane can reduce your gas mileage and can lead to damage to valves and piston tops.

Use what is specified by the car’s manufacturer!

If your owners manual allows a lower octane than you can change without worry. The engine doesn’t get used to a specific octane. As long as the minimum ocatne is met the engine is happy.

What does your owner’s manual say? Why have you been using 91 octane so far?

The manual calls for premium…but California has very high gas prices and saving at the pump is very common…nowadays

The manual probably uses one of two words.

  • “recommended” means you can likely go below 91 without any worries (you listen for pinging and change back if you get it frequently).

  • “required” means that you should do the math - because the little bit extra on gas is way cheaper than the really expensive engine work you’ll end up needing. I.e. stay with the premium. Actually if you do the math on the prices you’ll likely be surprised at how little difference there is in the end between the higher & lower octanes.

I don’t believe it is a good idea to attempt to save money by using a cheaper fuel than what the manufacturer said it requires.

If you can’t afford the correct fuel, then you surely won’t be able to afford the forthcoming problems that using the wrong gasoline may cause.

Depending on what your mileage is right now, and you get lower mileage with the 87 octane, are you REALLY saving any money by filling up more often with 87 than 91?

Every one that responded…thanks…point made !