I reciently ran several tanks of 89 octane in my 5.7 and did not see any performance gain or milage gain.I have always used 87 in my 300c with no problems and I am always concerned about the performance of my autos. I own a 12 second 440 powered 37 Dodge Street Rod and my son has a 10 second 440 powered 65 Dodge. I reciently took a trip of several hundred miles each way that I take several times a year using 89 and got the same millage and performance as with 87. So with a 5.7 don't wast money on 89.