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I've heard from some people that high octane fuel will increase gas mileage. Around here, we have the basic unleaded(87), mid-grade(89), and premium(93 or 91). I've been using the basic 87 unleaded forever because it was the cheapest. Also, in my car's manual it recommend the lowest grade to use is 87(Grand Am 2004 V6)

On a car with no modifications will high octane fuel in general improve gas mileage, decrease it, or have no effect?

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8 Answers 8

up vote 22 down vote accepted

Use the recommended gas for your car. Going lower than the recommended may reduce fuel economy as the engine may have to retard timing to avoid detonation. Going higher than recommended won't help as your engine is unable to take full advantage of it, as well as the fact that higher octane fuels actually contain slightly less energy (they just offer a more controlled burn that higher compression engines can take advantage of).

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I have a 1998 Subaru Forester with the standard engine (87 octane recommended). It started throwing a 420 code repeatedly, indicating that the catalytic converter was failing. On the advice of a trusted mechanic, I started using 92 octane fuel. He said that the car's computer would automatically make adjustments to run hotter/leaner and that the higher EGT might help to clean out the cat. The 420 code stopped recurring after only one or two tanks of 92 and has not returned. Might have saved me at least $1000. –  Miles Erickson Dec 30 '11 at 15:18
    
Octane rating is not necessarily correlated with energy density. That varies from one batch of gasoline to another depending on the refiner. If ethanol is blended to achieve a higher octane rating then yes, the energy density will be lower, but don't take that for granted. –  Nathan L Dec 12 '13 at 15:08

In my experience the difference is marginal at best. Given that I have known many people who swear by the huge benefits of high octane fuel, I've tested the hypothesis on my own vehicles several times.

Each time I've tried the experiment (first with a 125cc motorbike, then with a 1100cc car, then with a 650cc sports bike and most recently with a 600cc commuter bike) the results have been the same.

The average miles done on each tank (I always run full to reserve so calculating mpg is easy) has usually increased, but by an insignificant amount and the extra mileage was always less than the extra cost of the fuel. For instance, on my current bike, it adds about 5 miles to the 200 mile full tank range (so 2.5% benefit), but adds 5% to the cost of fuel (£23 per tank rather than £22)!

I believe that I have also noticed some benefit in terms of performance (acceleration), but only on some of those vehicles and I fully recognise that this as likely to be due to confirmation bias (thanks Bob) as it would be to any real benefit.

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I would recommend that anyone interested in fuel economy just measure it on a regular basis. Use your trip odometer and zero it at each fill up. Drive regularly for a while and then refill. Divide the miles driven by gallons consumed. Repeat as necessary until your mileage improves. –  Bob Cross Jul 26 '11 at 22:01
    
@Bob Yup, that's what I do too. Always filling up to a full fuel tank means that the amount of fuel that that you fill up with is also the amount of fuel you've used since the last fill up and thus last trip reset - hence my comment about making it easy to calculate mpg. Normally though I don't bother with the calculation, I just know that my vehicle should get about 200 miles to the tank. –  Mark Booth Jul 27 '11 at 8:52
    
Yes I also tested it with my car after asking this and ended up getting about 24MPG with 87 and 25MPG with 91. So very marginal. –  Earlz Jul 30 '11 at 21:49
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@Earlz, yes, I would conclude that that 1 mpg difference is arguably within the measurement error bounds. –  Bob Cross Mar 12 '13 at 19:52

Some aspects of this question have been covered previously. In my previous answer, I pointed out that using octane that is too low will lead to retarded timing and higher emissions.

With respect to fuel economy, you can never trust someone's casual anecdote about fuel economy. As Mark points out, they're almost certainly going to be operating under a case of serious confirmation bias. Fuel economy is something that has to be measured scientifically to establish that, under the same conditions with the same vehicle, fuel X is better than fuel Y by this amount, thereby saving you a net of Z dollars over time T.

If the person you're talking to can't quote values for all of those variables, you're just having a cocktail party conversation. Don't take it to seriously.

In short, octane that is too low is bad. Octane that is too high is possibly a waste.

My favorite fuel economy example continues to be the episode of Top Gear where Jeremy Clarkson clearly demonstrates that a BMW M3 is more fuel efficient than a Toyota Prius. There's no question: both cars drove the same course at the same speed over the same distance and the Prius used more gas.

Admittedly, he was driving around a race track and the Prius was going "flat out" (sarcasm quotes for the Prius) while the M3 was just loping along.

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+1 confirmation bias, that's the name of the bias I was looking for. *8') –  Mark Booth Jul 26 '11 at 16:07
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@Mark, you were right - if you look, confirmation bias shows up on your original link. It's just the specific bias of a whole set. It's awful - you can't even trust yourself a lot of the time.... –  Bob Cross Jul 26 '11 at 21:59

Well the first thing that you should do is re-read your manual. If the manual specifically says to use the lowest grade use that, the maker knows best. However if it says that the cars were made for 87 or higher your car may be able to take advantage of the higher octane, such is the case with many subarus. However the only way that you will truely be able to tell if it will increase your mileage is to test it. Instead of filling up an entire tank with high octane, i would suggest making a trip to your local Dollar store. In most dollar stores they have a bottle of octane booster, this, obviously, is not going to be anything like a premium blend of fuel, however it may increase your gas mileage as it did for mine. If it does so, though it may be a small amount, you may want to consider trying a premium or super blend of fuel. worst thing that will happen is that it doesnt work, but it could be worth a try if your car is able to use it and increase you mileage.

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It may be worth trying out a tank of higher octane once, paying attention to how long it lasts over a tank of what you've normally been using.

I noticed my 2001 Lancer went further on a tank if I used higher octane. It wasn't a lot, but enough that it was obvious: I routinely use the trip meter to measure how far a tank goes, and don't normally fill it until it's quite low. I suspect the EMU was detecting the slightly different output from higher rating fuel and leaning the mixture to compensate. Since your car is newer, it may well do the same thing.

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2008 enclave I get about 15 mpg in town on 87 and about 18 in town (if I drive it if my wife is driving it don't matter she gets about 11 mpg on either I try and explain to her if you won't take off hard it would help but she has to get up to the speed limit as fast as possible point being watch your driving habits when you change your gas octane it can make a difference but if you trying to save at the pump drive easier it really helps

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I have a 2009 Dodge Challenger SE (V6). The compression is 10.5-1. I used 87 octane for awhile and then switched to 89. My performance improved and my MPG went up by 2 miles in city driving and 3 MPG on the highway. I have stayed with 89 octane since. It has always been told to me that the higher the compression ratio of the engine required a higher octane. This seems to be true with this car as the cost per mile is now lower overall. I am currently running the test on the wife's new 2013 Chevrolet Equinox that has even a higher compression ratio.

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It's like this...

Higher octane fuels allow the compression ratio of the engine to be higher, and the higher the compression ratio, the more efficient the engine - this is the principle behind diesel engines, in fact.

That said, the compression ratio of most engines is not allowed to change significantly, so if your engine has a 10:1 compression ratio, you will NOT get better fuel economy with a higher octane fuel - the compression ratio is already set, and it is this parameter which affects efficiency. The exact same engine using a 13:1 compression ratio for instance WILL be more efficient, but it will require higher octane fuel.

Thus, lower compression ratio engines will have a lower efficiency, but they can use lower octane fuels which are cheaper. High compression ratio engines will be more efficient, but they require higher octane fuels which are more expensive. Make sense?

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protected by Larry Dec 12 '13 at 4:23

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