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Octane ratings

Fuels Octane. Most car manuals specify 87 octane rated fuel or higher. In North Dakota for most brands the lowest grade, regular, is 87 and +(plus) is 10% ethanol and 89 Premium is sometimes 91 or 93. Some brands have ethanol in all 3 grades. In western Montana the regular is still 87 but as you move towards Billings it drops to 85.5 and in Idaho it is all 85. I’ve heard it said it is because of the altitude but in Colorado, Oregon & Washington it is all 87 so that argument is weak. Is it just designed to force higher octane grade purchases at higher prices. Driving experience seems to indicate up or down hill is the primary arbiter of mileage not octane. In years past I remember gassing up in Reno Nevada and getting 100 Octane fuel which seemed to give better mileage across NV. I also note that some states require notice at the pump if ethanol is added other states do not. That can cause problems with cars that have never used any ethanol based fuels as the fuel filters can become plugged from a single tankful. Thanks

Is there a question anywhere in that paragraph?

You wouldn’t have gotten better mileage with 100 octane gas…it was just pricy fuel that actually has less energy.

In NY we have 87, 89, and 91 at the pumps, at least the ones I go to.

A quick google search found an explanation for the regional variations on Wikipedia:

Here’s a brief explanation: due to the thinner air at higher altitudes, less air is pulled into the engine. This means there’s less compression in the chamber, and that leads to less knock. A higher octane isn’t necessary then. Makes sense to me, but I’m not an expert by any stretch of the imagination…just an environmental engineer. :slight_smile:

The post is a bit rambling, and I was unable to find a question, however I will respond to one point.

10% ethenol will not hurt your car. 85% ethenol used in any car whose fuel system is not designed for it will hurt your car. The ethenol will attack some fittings. You can tell if your car is designed for it (they’re called “flex-fuel vehicles”) by looking on your owner’s manual.

I am also searching for a question regarding octane (or even octaine), but I can’t seem to find a question in the original post. As to the lower octane numbers posted on pumps at higher altitude, it is due to a lower octane requirement for engines at higher altitudes. Contrary to some people’s beliefs, this is not a conspiracy to force consumers to buy more expensive gas.

And, as was already stated, gas that is of an higher octane than what the engine requires will not produce better fuel economy.

hoffmair went to wikipedia, as did I. This article tells you as much about gasoline, and similar fuels, as most people want: Click on this Enjoy.

“I also note that some states require notice at the pump if ethanol is added other states do not.”

Many of these laws are simply left over from an earlier generation when drivers were reluctant to use gasoline containing ethanol. There probably is no longer a need for such a notice, but state legislators move slowly, particularly if existing notification laws do no harm.

Don’t know where you have been driving in Colorado, but along the front range (and in the mountains) regular is 85 octane.

What I can’t find out is what octane I should be using. Back in 2004 when my Maxima was new the recommended fuel was premium’ which was 87 octane. 87 is no longer premium fuel, but can I still use it?

“premium” means whatever is labeled “premium” on the pump, and will change as a function of location because of the reasons listed above. Where I live, that’s 91-93 octane. In some places, it’s 89. I don’t recall ever seeing 87 labeled as premium.

BTW, usually 100 octane pump gas is labeled “for offroad use only” because it’s race gas, and has lead in it, which will hurt your emissions system. Don’t use it.

ALL of the regular sold in The Inter-mountain West is 85 octane…That includes all of Colorado…You have to pay .10 cents more to get 87 octane, which the oil companies sell as regular in the rest of the country…

Note: Higher octane does not mean more power nor does it mean better mileage. Anything over the recommended octane is likely to be a total waste of your hard earned cash.

If the owner’s manual “RECOMENDS” high octane, not using it could reduce you mileage. If it “REQUIRES” it and you fail to use it, it could damage your engine.