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Discussion Starter #1
So I've heard on this forum that mid-grade and premium fuels make your car run better or at least make the engine cleaner. But, just days ago, everyone now says that 87 octane (regular) is best for this car and premium is not worth it and "makes it run worse". Soooo... which one is it?
 

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I don't think the premium fuels will make our cars run worse. I think that they just don't offer much of an advantage for the added cost. I've also noticed that the mpg estimate typically goes down with higher octane fuels.

This is just my observations from my personal experience though.
CK
 

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I've tried higher grades a couple of times. Not worth the extra cost. The car just doesn't seem to run as good for some reason, and mpg drops a couple mpg. It doesn't seem like higher octane fuel would do that but more than a few others have reporter similar results.
 

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Discussion Starter #4
Does higher octane at least help reduce carbon deposits or am I throwing my extra 20 cents/gallon down the drain?
 

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Does higher octane at least help reduce carbon deposits or am I throwing my extra 20 cents/gallon down the drain?
if you have the skyactiv engine, then the fuel can't reduce carbon, as it doesn't enter the engine through the valves, it's injected directly into the cylinder - so it's a waste of money
 

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Only if your car is tuned to run 91, otherwise stay with 87 if you’re stock and you’ll be perfectly fine.


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please remember that RON numbers are different from country to country

USE WHATEVER YOUR MANUAL SUGGESTS

and ignore totally whatever your stealership recommends - they sell cars, they don't engineer them
 

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From what I was always told, use what the manual says. The mid-grade and higher gas is only good for cars that have engines designed to run that fuel. You will get worse gas mileage if your engine wasn't designed for that grade. The premium fuel usually has more of the detergents in it so it might help keep your engine cleaner, but you will pay a lot more for it and most likely get worse mileage.
 

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Agreed...use what the manual requires....BUT I have felt a difference when using 91 SHELL. I drive I95 constantly at high speeds at times, yes it burns up faster but the engine runs smoother, If you try it yourself you'll notice subtle changes. I run SHELL 87 on a regular basis but I will use and fill up with the V power 91 now and then. Just try it yourself its a few dollars diff...
 

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Does higher octane at least help reduce carbon deposits or am I throwing my extra 20 cents/gallon down the drain?
Probably not because these cars are Direct Injection, all the extra detergents are useless anyway. Pretty much the only thing you can do (to my understanding) is to run tubing to a catch can between the Postive Crankcase Ventilation and the intake return inlet. I have a JBR one on mine, and the only extra maintenance step is draining the catch can every once in a while (2-3month intervals).

It supposedly helps condense and catch unburnt fuel/oil/debris from re-entering (and gumming up) the intake manifold and pistons. Someone can feel free to correct me if I'm wrong though, as that I only know about all of this because taking care of my car is a hobby of mine, and I am not a professional mechanic.


Edit: did not pay attention to the model years, lol. The 3rd gens are DI, but I don't have a clue if 2nd gens are or aren't. If they aren't direct injeted, then I'd imagine that the extra detergents in premium would help keep the engine cleaner.
 

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You can help clean carbon deposits by running your car to redline once in a while or running higher RPMs for prolonged periods (look up "Italian tune-up"). Mazda designed the cylinder head to keep the intake valves hot, such that they burn away the deposits. Of course aftermarket companies will tell you that installing their catch-can is a must...
 

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What is really important is running quality fuel with the right amount and the right mixture of additives, e.g. using only Top Tier rated fuel in the United States (do not know if Top Tier extends beyond).
 

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So I've heard on this forum that mid-grade and premium fuels make your car run better or at least make the engine cleaner. But, just days ago, everyone now says that 87 octane (regular) is best for this car and premium is not worth it and "makes it run worse". Soooo... which one is it?
It depends on the car. Some cars (e.g. my father-in-law's Z06 Corvette) are designed for premium. The Mazda 3 is perfectly happy with regular. Many people have reported here that they tried higher grade and they lost power and mileage. They went back to regular and the car started acting fine again.

What is really important is running quality fuel with the right amount and the right mixture of additives, e.g. using only Top Tier rated fuel in the United States (do not know if Top Tier extends beyond).
Agreed. Regular octane, as long as it's high quality, is perfect for the 3. I'd say find a local gas station with reasonably priced, Top Tier fuel and stick with it so you always know what you're getting.

I have found a Conoco station near my house where I always fill up. It's one of the cheaper stations in my area, but it's high quality (Top Tier). I get the cheapest grade (85 octane, but I'm at high altitude in Denver so it's fine). My 3 always runs great and I've averaged 35+ mpg over the past 6 months since I've been filling up there, and that's even with the 2.5L engine.
 
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