Gas situation.. need your thought.

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I'm tired as hell and have to get up early so I'm posting this as is... If there are any inconsistencies I'll correct 'em tomorrow at some point.

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No, adding pretty much any fuel stabilizer will reduce your mileage.

The only way that some vehicles see improved mileage from higher octane fuels is with modern computer controlled valve timing, being able to lean out the fuel mixture by selectively increasing the amount of gas fed back via the EGR system and a much broader control over the ignition advance. EGR control also allows a virtual reduction in engine displacement. Exhaust gas has nothing in it to burn, it's inert. So, pump 1 liter into a 2 liter engine each cycle along with the 1 liter of viable mixture and PRESTO! You're now driving a 1 liter engine! And it's running much cooler so you can inject a little less fuel too... BAM! 1 liter, low consumption. BUT when you stomp on the throttle it's miraculously a rich running 2 liter turbo charged monster leaping from 40hp to 300hp... 60MPG to 10MPG. Not a real world example but not an exaggeration. This is what many auto manufacturers have done to achieve some of the wicked mileage you see on reasonably powerful cars today. You run a lower octane in these newer cars and they take a hit on both ends... can't run so lean and can't push as hard.

For most common pre2000 cars you can forget the thought of higher octane improving your mileage unless you're running too low of an octane already. Run too low and you get detonation/ping, the ECU pulls timing and ADDS more fuel to get the engine running safely again and thus you get worse mileage. Again: It's a balancing act.

Higher octane is SAFE... And pretty cheap insurance, but there are reasons why you may not want to run it. On my MKIII I ran an AFR of 16.1:1 at lean cruise, I think about 15.3:1 regular closed loop cruise and I think the idle was 15:1 or so. On a road trip my mileage on 87 octane was crazy. Of course it was also tuned for some performance and the minute I started driving aggressively 30% throttle or higher it would go to 13:1 and richer and richer as it started boosting and my mileage would plummet. :P Now on this car with a standalone or advanced fuel piggyback you don't want to run so lean if you're also running low octane fuel... balance. You also do NOT want to run a very lean cruise if your EGR is not functional and unobstructed... you NEED that extra cylinder cooling. I was running the MAFT Pro, for those who are going to ask.