So, For the longest time I've been running Hemifevers 87 Octane on my 5.9. No AC, Windows always down, with the downstream 02 sensor showing as bad (it was stuck open.) Performance felt fine to me.
This past week I replaced the O2 sensor and no longer have any check engine light. Performance feels the same, if not slightly more sluggish overall? (Placebo?)

Heres the kicker though, I went from getting 11-12mpg and throttling hard whenever I wanted to have some fun/ all over the place :P now I'm seeing 10.5ish if I'm mostly gentle on the throttle, with 10.2mpg at my most recent fill up. Any thinking on why/if I should be seeing higher fuel consumption? If its the O2, kinda makes me want to swap back to gain the 1-2mpg for almost no performance loss (possibly gain?!)...

Anyways, I had a mini-theory, so I went ahead and switchd over to the 89 octane tune instead of the 87. I know higher octane tunes just advance timing to compensate for the added headroom of higher octane fuels to prevent early detenation. Since im still using 87 fuel on 89 tune, should I be fine or should I worry about pinging/early detonation?

 

That may be a problem...... drive it, and see what happens. If you get a lot of pinging, might add some octane booster.

 

Just drove it and I wasn't hearing or noticing any pinging that I could tell (cant roll down windows because just got them tinted). I'll try to listen more closely the next time im out, but it sounded the same as normal. I may just go back to 87 though, it doesnt feel like im getting better power/pedal so I dont think itll improve mpg ;( Guess im just stuck around 10 :/

 

Even my 3/4 ton 4x4 would do better than that.... How old/what brand is the front O2?

 

I'm not sure. Its pretty old though, truck sat without an engine for about two years in driveway with it in, and that was way back in 2015. I just went on a short trip to the store, and uplugged it prior. It ran more sluggish, lots and lots of wind sound under throttle like it was really sucking air (or pushing fuel maybe??) so im assuming the O2 more or less works, but it may not be good, or working perfectly. Some of the loud air is also because I removed the silencer fins on my throttle body. Do you think running AC full blast every time I drive vs windows down could reduce it to that low?

If it helps, truck is a 5.9 1500 sport standard cab with the tune, CAI, shorty headers, and pipe dumps about 1ft after cat (idiot who had it before it did some crap half *** welds so we just cut it back a bit to stop it leaking out his rusted welds.).

 

Single exhaust? One cat? What kind of shape is the cat in? Might try physically removing the front O2, and see if it runs any better.

 

Yeah, just the stock Y pipe and stock cat. I'm assuming the cat is fine since I don't have any codes

 

Cat won't necessarily set any codes...... especially if it is clogged.

 

Ah, I have no other reasons to believe its clogged however, as performance is overall good. Its just the low mpg since I got AC and fixing the rear O2 sensor.

 

Might want to replace the front O2 sensor... performance degrades over time, and that will affect gas mileage.