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This thread has been going on long enough that I'd like to comment on some of the posts. As to anecdotal reports like "my car just runs better on premium", to those I just have to say that it's all in your head. You may've gotten a tank of 'bad gas' that was adulterated by condensation, but the octane rating had nothing to do with that.

As to the guy who says his car "runs cleaner"; What? You took it for an emissions test and quantified that? Really???? Octane ratings do NOTHING for emissions. Nothing at all.

Premium has more power? No, it does not. Premium resists detonation longer, that's all. It actually contains slightly LESS 'power' per gallon than regular does. All premium does is allows higher-compression motors to extract more 'power' out of fuel without destroying the piston crowns with excessive per-detonation.

Ethanol-free gas: Older engines often have old seals and gaskets not designed to deal with the corrosive aspects of ethanol. Small gas appliances as well. Gasoline and air mix at about 18:1 ratio, ethanol and air at about a 15:1 ratio, so, the less ethanol in your fuel, the better fuel economy. Expect about a 2-3% improvement; not enough to make the $.50/gallon (or worse) extra cost be worth it. Also, many places with ethanol-free gas only offer it in premium grade. Others offer it as a 90 octane mid-grade. I buy a couple gallons of that each year for my lawnmower (also reduced 'vapor lock', but that's a problem with the air-cooled lawnmower motor that my car doesn't have).

In short, if you still believe that putting premium into your unmodified Impreza, then you are a sucker. Instead of wasting an extra $5 on each fill-up, just throw it out the window or flush it down the toilet; you'll get zero benefit from your money either way. Save the premium gas for your high-compression sports cars, your old muscle cars, and the like. They need it, you do not.
 

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2019 Subaru Impreza Sport 5 Dr. Manual Shift
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Personally I use the fuel they say in the owners manual I've never had an issue. The only time I've used anything different is when I had a cobb accesport on my Forester XT and I could set it to use 100 or 110 octane I got at the drag strip.

This is worth a watch.

 

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A;so, I'd like to mention that the ECU is 'mapped' to work properly only on 87 octane. Cars which take premium are 'mapped' to work with the higher octane fuel. I once had a 1986 Omni Turbo with a less-than-legal ECU that was mapped for something like 97 octane, and I ended up having to mix 94 octane Sunoco gas 50%/50% with 100 octane racing fuel. But then, this car's ECU had been modified to produce considerably more turbo boost than stock (and was almost certainly not street-legal). I believe the previous owner had ran it on aviaton gas. I ended up putting the catalytic converter he'd removed back on the car, which actually made it run better.
 

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2019 Subaru Impreza Sport 5 Dr. Manual Shift
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Our 2006 Tribeca, 3.0 H6 had premium fuel as "Recommended" not required you were able to run regular fuel. The ECU would adjust accordingly. The owners manual said for best performance and MPG's use premium. Performance wise honestly it was hard to tell any difference. It was not a light vehicle. We took a 14 hr drive loaded down with kids and gear. Going I used premium and returning I used regular. The amount of MPG increase vs the cost of the fuel s pretty much a wash. It's the only Subaru I've owned that had a recommended statement. Later they took that motor, made it a 3.6 and mapped it strictly for regular fuel. That was our Legacy 3.6R
 

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2019 Impreza Premium Hatch CVT
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I use 85 octane. I have for the last ~20 years that I’ve lived at high elevation. No pinging or knocking on any of my vehicles totaling a combined 400,000 miles on them. I typically run a car to about 200k before selling them.

The Impreza is averaging just under 34 mpg.
 

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Around 1999, Toyota made a 3.0l V-6, that it put in both the Camry and the Lexus GS. The Camry was recommended to use regular, the Lexus premium. The Lexus got 20hp better than the Camry, on the identical same motor; only the ECU was different. A lot of Camry owners started looking for Lexus ECU's at junkyards. I'm still not certain if an extra 40 cents a gallon was worth it just for an extra 20 hp (and on a Camry, yet!). I know that fuel economy was also just a bit worse with the switch.
 
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