A good friend of mine has a 1999 Mitsubishi Galant gtz or something like that with a 3.0 v-6. The manual calls for 91 octane but as gasoline prices keep going up higher and higher with no end in sight, can she change to the next lower octane?

This topic has been hashed out here MANY times. You could easily find it by using Search in the red bar at the top of the page.

Does the owner’s manual say that 91 octane is REQUIRED or RECOMMENDED? There is a difference. If it’s required, she should use it. If it’s only recommended, then the knock sensor of the engine’s computer should be able to retard the timing of the spark to allay any potential damage. She may get away with using lower octane where it’s REQUIRED for a while, but it will eventually damage the engine, and performance will definitely be lower. It’s a “you can pay me now or you can pay me later” sort of thing.

The owner's manual will say that it must have high octane or that it may use regular but should use high octane.  The parts the leave out are:
  • If it needs high octane, using regular can damage the engine.

  • If it should use high octane then it has a system to sense the low octane and to make adjustments to the car that will protect the engine. However the adjustments will result in lower mileage (reducing or wiping out any economic advantage) and will reduce the power, defeating the reason most people buy cars that require high octane.

  • Using high octane in a car that does not recommend it usually does nothing but cost you more and in a few cases can cause damage.

Thanks for the input. Its better when its in writing.