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Discussion Starter · #1 ·
Is there any difference in MPG when you use 91 vs 87 gas?

I'm currently only using the 87 octane at the gas station for obvious reasons. The gas price is here in the SF Bay area around $2.51 although I got gas last week for $2.41 at Costo. I get roughly 52 MPG with freeway miles. Not bad.

Has anyone checked out to see if there's a difference between the 87 and 91 octane or perhaps the 89.

Thanks for replying. Happy Prius driving :D
 

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Unless you have the VERY unusual situation where you have knocking robbing you of power, NO!. Actually, you will lose some MPG, as higher octane has LESS energy in it for the additives that increase the octane.

Octane rating is the resistance to self detonate at high compression, not the content of energy.
 

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Discussion Starter · #3 ·
Cool, so I don't need to buy any exspensive gas to run.

There are certain car companies that claim that their car needs the best octane level (91) to make their car run well.

So would the car then run even better if you live in a state where they sell octane #85? unfortunately california does not have this.

Thanks
 

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DanMan or someone can probably elaborate more eloquently, but some engines REQUIRE the higher octane fuel to run well -- the Prius' engine is not designed that way. I was also looking at the Acura RSX Type S and one reason I didn't go that way was the need for high octane fuel. It has something to do with high-HP gas engines and how they run.
 

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The only times I've ever used higher octane gas is on older cars that noticeably struggle and "knock" if lower octane gas is used. It made a definite, noticeable difference in my 1985 Ford Tempo and a 1983 Dodge pickup, both over 10 years old at the time I drove them.

On a Prius, you don't need the higher octane.
 

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Yes, some engines do require higher octane because they have higher compression ratios on an Otto cycle engine. Because the compression is so high, regular 87 octane would pre-ignite and cause pinging, power loss due to igniting too soon, and probably eventual engine damage. You want the spark plug to determine the time to ignite, not subtle random events that can't be controlled.

Hi compression yields more power mostly because of the longer power stroke in the 4 cycle engine. Since a longer stroke means more air/fuel mixture being shrunk to a smaller space, you have higher compression.
Some alternative engine systems, such as our Atkinson (some think it more of a miller) reduce the compression while still having a longer power stroke by reducing the intake stroke, or releasing some of the mixture taken in during the intake stroke during the early part of the compression stroke.
 

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Read the owner's manual, use the octane rating listed for the car. For the Prius in the US, that's 87 octane ((R=M)/2).

The only reason to use a higher octane fuel than "regular":
1. your owner's manual says to use higher (not the Prius, but some performance cars will want higher octane fuel)
2. your car's engine is knocking
3. it's the only way to get low-sulfur fuel in your area (sulfur will slowly poison the catalysts, making your car's emissions worse as time/buildup progresses)
4. you like giving your local gas station extra money

Higher octane fuels actually have lower BTUs (less energy) than a lower octane fuel. So, using a higher octane fuel in a car not designed for it (such as the Prius) will actually give you lower MPG. (In a car that requires the high-octane fuel, going to a lower octane will lower MPG, though.)

Added to that, is that the Prius' Atkinson/Miller cycle engine is known to give check engine lights (engine misfire codes usually) when people use the high octane fuels...

I also suggest: http://www.ftc.gov/bcp/conline/pubs/autos/octane.htm
<quote>
Facts for Consumers: The Low-Down on High Octane Gasoline

Are you tempted to buy a high octane gasoline for your car because you want to improve its performance? If so, take note: the recommended gasoline for most cars is regular octane. In fact, in most cases, using a higher octane gasoline than your owner's manual recommends offers absolutely no benefit. It won't make your car perform better, go faster, get better mileage or run cleaner. Your best bet: listen to your owner's manual.

The only time you might need to switch to a higher octane level is if your car engine knocks when you use the recommended fuel. This happens to a small percentage of cars.

Unless your engine is knocking, buying higher octane gasoline is a waste of money, too. Premium gas costs 15 to 20 cents per gallon more than regular. That can add up to $100 or more a year in extra costs. Studies indicate that altogether, drivers may be spending hundreds of millions of dollars each year for higher octane gas than they need.

What are octane ratings?
Octane ratings measure a gasoline's ability to resist engine knock, a rattling or pinging sound that results from premature ignition of the compressed fuel-air mixture in one or more cylinders. Most gas stations offer three octane grades: regular (usually 87 octane), mid-grade (usually 89 octane) and premium (usually 92 or 93). The ratings must be posted on bright yellow stickers on each gasoline pump.

What's the right octane level for your car?
Check your owner's manual to determine the right octane level for your car. Regular octane is recommended for most cars. However, some cars with high compression engines, like sports cars and certain luxury cars, need mid-grade or premium gasoline to prevent knock.

How can you tell if you're using the right octane level? Listen to your car's engine. If it doesn't knock when you use the recommended octane, you're using the right grade of gasoline.

Will higher octane gasoline clean your engine better?
As a rule, high octane gasoline does not outperform regular octane in preventing engine deposits from forming, in removing them, or in cleaning your car's engine. In fact, the U.S. Environmental Protection Agency requires that all octane grades of all brands of gasoline contain engine cleaning detergent additives to protect against the build-up of harmful levels of engine deposits during the expected life of your car.

Should you ever switch to a higher octane gasoline?
A few car engines may knock or ping - even if you use the recommended octane. If this happens, try switching to the next highest octane grade. In many cases, switching to the mid-grade or premium-grade gasoline will eliminate the knock. If the knocking or pinging continues after one or two fill-ups, you may need a tune-up or some other repair. After that work is done, go back to the lowest octane grade at which your engine runs without knocking.

Is knocking harmful?
Occasional light knocking or pinging won't harm your engine, and doesn't indicate a need for higher octane. But don't ignore severe knocking. A heavy or persistent knock can lead to engine damage.

Is all "premium" or "regular" gasoline the same?
The octane rating of gasoline marked "premium" or "regular" is not consistent across the country. One state may require a minimum octane rating of 92 for all premium gasoline, while another may allow 90 octane to be called premium. To make sure you know what you're buying, check the octane rating on the yellow sticker on the gas pump instead of relying on the name "premium" or "regular."

For More Information
If you're concerned about the accuracy of an octane label - or if you don't see a yellow octane sticker on a gasoline pump, write: Consumer Response Center, Federal Trade Commission, Washington, DC 20580.

The FTC works for the consumer to prevent fraudulent, deceptive and unfair business practices in the marketplace and to provide information to help consumers spot, stop and avoid them. To file a complaint or to get free information on consumer issues, visit http://www.ftc.gov or call toll-free, 1-877-FTC-HELP (1-877-382-4357); TTY: 1-866-653-4261. The FTC enters Internet, telemarketing, identity theft and other fraud-related complaints into Consumer Sentinel, a secure, online database available to hundreds of civil and criminal law enforcement agencies in the U.S. and abroad.

</quote>
 

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The octane rating is the average of the Research Octane Number (RON) and the Motor Octane Number (MON). Iso-octane has a rating of 100 and normal heptane has a rating of 0 so premium at 91 is equivalent to 91% iso-octane and 9% heptane in terms of energy.
 

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Discussion Starter · #9 ·
MRV,

wow you realy now your stuff

thanks for all that data. 87 octane it is.

Happy prius driving :D
 

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Is knocking harmful?
Occasional light knocking won't harm your engines, and doesn't indicate a need for higher octane. But don't ignore severe knocking. A heavy or persistent knock can lead to engine damage.

IMHO, the knocking does not harm today's engines (due to better design than old ones), however, unthoroughly burnt exhaust would harm your CAT(cat. converter) and gradually shorten its life.

Come to think of it, most cars either recommend 87 or 91, who is buying 89 octane anyway?!

In some other countries, the rating system is different (not (R+M)/2). Their 91 could be our 87 here. Their 97 is close to our 91. Make sure you know that when you travel abroad.
 
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