I just purchased a 03 mach1 with a five speed two weeks ago when i traded in my 03 GT. I had been running 93 octane like the dealer told me to with a DOHC motor but it is so expensive so I tried 89 octane to try to save money and my car is running much stronger for some reason now. I just wonder if any body else has had the same experience. I dont want do damage the engine. Thanks for any input. Oh and I wish I had bought a mach1 long ago, this thing will smoke my 03 GT I used to have . What a awesome car!