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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?
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).
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.
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.
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.
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.
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?
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
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.
Holding all else equal, higher-octane fuel contains more energy per unit volume, and hence will give you higher "mileage." But on average it's only 0.7% higher, which is not enough for you to notice, and certainly not enough to justify a price premium (of more than a percent). From the gasoline bible:
Heating values can be used as surrogates for actual fuel economy measurements when considering the effect of gasoline composition on fuel economy. Conventional fuels always have varied in heating value. One cause is the formulation differences among batches and among refiners. A survey of 1990–1991 conventional gasolines found that the heating value of summer gasolines varied over an 8 percent range [which affects MPG by the roughly the same proportion]. Heating value also varies by grade and by season. On average, the heating value of premium-grade gasoline is about 0.7 percent higher than regular grade because premium grade, in general, contains more aromatic hydrocarbons, the class of hydrocarbons with the highest densities. The heating value of winter gasoline is about 1.5 percent lower than summer gasoline because winter gasoline contains more volatile, less dense hydrocarbons.
