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Discussion Starter #1
I have the 3815 superchips flashpaq. 5.9L. I have it programmed to the 91 performance tune right now with a tank full of 93 octane gas. I want to program it to the mileage xs tune but idk if it is going to mess anything up being that it is running 93 octane. Can/should i run 89 with the mileage tune or should i stick to 87? I was just going to wait until i got to around 1/4 tank and fill up with 87 and then re-flash it.
 

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If you tune your ECU to 91, and run 93...The ECU is going to adjust itself because of the higher octane level, and could cause it to run poorly...And I would go with 87 because its cheaper, and then tune it again...Switching to the 89 octane will not show much of a difference in mpg even though you tune it, compared to the 87 when tuning...
 
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will it still run well if i use 87 with the mileage tune?
Not as good as the 91 tune and my experience with my truck indicates that it will pre-detonate a bit with 87, especially when warm outside or pulling a load up a hill.

I actually get better mileage with 89 vs. 87.

Keep in mind, I have no tuner on my truck and won't get a tune until after I build the motor.
 

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Discussion Starter #6
ok i found out that my truck being that it is a 1998, it is recommended from the factory that you use 89 octane. When you tune it to the mileage tune it actually says on the programmer that you can use 87 and up. I filled up with 89 cause it runs better with it. however..there is a seperate tune that you can run 87 with. it is the 87 octane tune lol imagine that right? but you can run 87 and it takes care of the pre-det ping. just wanted to let everyone know.
 
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