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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.
 

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I would use 89 octane. SOTP is not always a good guage. While 87 may work most of the time, there will be times that 89 is better, such as under WOT, very hot +90 degrees temps when pinging is most likely, or poor quality gas.

Here is from the owners manual.

FUEL REQUIREMENTS
2.7L Engines
Your vehicle is designed to meet all emissions regulations and provide satisfactory fuel economy and performance when using high quality unleaded gasoline having an octane of 87.

3.5L and 5.7L Engines
The 3.5L and 5.7L engines are designed to meet all emissions regulations and provide satisfactory fuel economy and performance when using high-quality unleaded gasoline with an octane rating of 87 to 89. The manufacturer recommends the use of 89-octane for optimum performance. The routine use of premium gasoline is not recommended. The use of premium gasoline will provide no benefit over highquality regular gasoline or mid-grade gasoline and in some circumstances may result in poorer performance.
 
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