I think this is very interesting.
I've been running a 87 octane tune for a month or so now. (consume a little more than one tank per week)
Well, at the same time I switched octane.. I went with these larger 32" tires which threw off my tune, odometer, speedometer, and scanguage. It took me a few days to get those lined out and now they match my GPS.
Well... my 87 octane tune/fuel is now running in the 17mpg area. I'm thinking it's not just the tires causing this drop in milage. The air temps are much cooler (30-40s) and I'm guessing by now we are into the full mix of winter fuel (lower stoich btw)
IMO that's horrible because I was running 20-21 avg during the summer and on shorter tires too.
Yesterday, I put in some speedway 92 octane. (tank was nearly dry - 19.4gallons to fill ) I then flashed my 93 octane tune into the trucks PCM, and dis-connected the battery to reset the KAM. Did the idle re-learn and then gave it a good 50 miles of stop/start/drive learn time before I started to accept the instant MPG numbers as factual.
My instant mpg went from 16-19 to 23-25! Timing running down the road at 70mph was 23-ish on the 87 tune (very very close to the OEM tunes timing)
My 93 octane tune is running in the 34-41 degree area depending on climb/flats/declining.
I can clearly see that when timing gets up over 35 the "load" goes down, the TPS is more closed (to maintain same speed), and the instant MPG is significantly higher. The sweet spot seems to be in the 36-41degrees area of timing. (The closer to 41degrees the better the milage)
The down side with this 92 octane fuel from speedway is that I'm hitting the knock sensor at heavy throttle. So I either need 93oct fuel, or I need to make a 92tune with reduced heavy throttle timing. (I get a decent discount at speedway that's why I'm trying it)
This kind of stuff is just interesting to me.