Who uses 89 octane gasoline?

I have never seen a car that uses 89 octane. All require 87 or 91. So, why is it sold? Are there actually some cars out there that do require it?

Also, how many storage tanks does a typical gas station have? Do they have three tanks, one for each octane rating, or just one for 87 and one for 91? It seems to me that they could be more efficient by using just two tanks, and using the pump to mix the two grades into 89 on the rare occasions when someone wants it.

My '03 Dodge Truck (5.7L V8) recommends 89 octane, although the manual states that you can use 87 if needed.

Of course, here in Utah most gas stations sell 85, 87 and 89 octane gas as the regular, mid-grade and premium grades, since the higher altitude requires less octane for equivalent performance.

They do actually mix the low octane and high octane gasolines together at the truck racks where the big tanker trucks fill up to get the mid octane gasoline.

If your car is pinging on the low octane gas, sometimes it just takes a move to the mid octane gas to get rid of the problem. A friend of mine with a ford explorer used 87 when he got it, and has moved up through 89 and now has to use 93 to keep it from pinging (its got 200k miles on it with no other problems, so its not so bad)

Most cars that “require” 91 octane gas will run perfectly well forever on the cheaper 89 octane.

Why do they “require” 91 octane then? Talk to a car buff. I’m just an owner of one such car.

About the tanks:


Interesting. I need to run 89 octane in my 2000 Dodge to keep it from pinging under load but Costco, where gas is cheapest in my area, only sells 87 and 91.

Mix it yourself at the pump, then. Half a tank of 87 and half a tank of 91 should get you right where you need to be.

As does BJ’s Club here in NY. Only I think it’s 87 and 93.

An engine made to run on 87, out of tune with carbon built up inside the combustion chamber, may need to be on 89 to resist pinging and detonation.

The carbon reduces the combustion chamber at top dead center, artificially increasing the effective compression ratio; also, the carbon will incandesce at operating temps, due to fuel pre-igniting on hotspots.

Make that “reduces the volume of the combustion chamber”

Working in the petroleum industry, I feel a need to set the record straight as to how most of this works. (Please keep in mind, I deal with the state of Oregon, and sometimes Washington) First, the wonderful (not) 89 octane gas is indeed a mixture of both 87 and 91. Usually it is one of two mixtures, either 90/10 (90% 87 and 10% 91, or 60/40). This mixture is done at the station, usually at the pump. Why would you mix this when filling a truck and waiste space, when you still have to carry the original ingredients to begin with. Why not carry more of the original ingredients and mix at the station? (It’s all about saving money) If you really want to see it yourself, next time you fill up, take a look around the top of the pump. You should see a couple of odometers (just like in your car that keep track of the milage), if your getting more than a couple of gallons, you’ll see one click over more than the other. Certain cars do run better on higher octain than other cars. Really it just depends on your car, and there are just to many variables to go into. As far as your second question, how many storage tanks do stations have, it depends on what they sell, and the volume of the station. If a store only does 2,000 gallons a day in gas, chances are, they are only going to have two tanks (1 for 87 and 1 for 91, providing they don’t sell diesel). If this same station does sell diesel, then they would have three tanks. If it’s a higher volume station they may have more than one tank for one product (two tanks for 87 and one tank for 91). Take a look the next time you pull in to get gas or diesel, you can usually locate the tanks by a metal “cap” that slightly protrudes from the ground. I would not advise you go and “inspect” these, for someone might think you are attempting to do something devious. I hope this answers your questions.

By the way, in another thread it was suggested (and cited in a convincing fashion) that cars with electronic fuel injection cannot in fact use “lower octane at higher altitude”.
That basically means that if you have anything newer than about '89 that requires 87 octange and live in the mountains, you really ought not use 85 octange.
Perhaps someone else can dig up the thread.

Usually it’s because the engine has a high compression ratio or has forced induction. The air inside the combustion chamber gets so hot, low-octane fuel would spontaneously combust. I have a Thunderbird Supercoupe which is supercharged and runs about 12 relative psi. If I put in 87 or 89 octane gasoline without pulling the blower belt or octane plug and floored it, I’d probably be picking up engine parts from the road.

Around here, it looks like 89 octane gas is indeed mixed at the pump. Whenever I’ve seen a gas station that has run out of 87 octane gas, invariably they’re out of 89 octane as well, leaving only the premium stuff.

What kind of car do you have? Modern cars can detect the knocking and pinging produced by low-octane gas, and retard the timing in order to run without damaging themselves. However, as I understand it, even if you don’t notice any difference, the engine is still running less efficiently, and developing less power. Usually, the decrease in gas mileage more than obliterates the few bucks you save by not using 91.

I have a BMW. Every time I fill it, I reset the trip odometer to track my gas mileage. I recently accidentally put 89 in it without realizing (because the pump was arranged 87-91-89, not 87-89-91). I didn’t notice any appreciable difference in power, and the engine ran fine, but my average mileage went down by about 6 mpg.

Hmm…these posts have me wondering if I should not use 87 octane in my car. I will make sure to read the manual, but just to see if any dopers know off the tops of their heads, it’s a '98 Subaru Legacy GT, 2.5 L engine. Since it’s the sportier version of the car, I wonder if it requirse a higher octane…that, and the check engine light is almost always on (it sometimes goes off after I fill up for a while) and the engine sounds…off. Not bad, mayube not a ping, just off. My neighbor thinks it’s the head gasket. Having almost zero knowledge of an engine, and not a lot of time/money to get it looked at, I’ll assume it’s not serious.

I have a Y2K Legacy GT, and I had almost the same problem. Turned out the “knock sensor” was faulty and thought my engine was pinging/knocking, so it was retarding the timing. I could definitely feel when it happened, I’d suddenly lose the acceleration for a brief instant, usually when going from a stop.

I don’t know how similar the 98 model is to the 00 version, engine wise. Since your check engine light is on, you may have a similar problem with a bad sensor, or maybe even an actual problem that a working sensor is detecting. I’ve had plenty of sensor issues with my car, the knock sensor was the 4th or 5th time I had to take the car in for the check engine light, and it was a different faulty sensor each time.

Just for reference, I know nothing about cars. I just looked up what a knock sensor was after they told me that it was the problem. My car sounds similar to yours so I thought I’d share.

I have a 1989 Cadillac which specifies “89 or higher” octane in the user manual. It does get better gas mileage with the higher octane gas, but not enough to make it cost effective. I get the best cost per mile using 89 octane.

Mrs Geek drives a Toyota Camry that uses 89 octane. It actually gets worse mileage using higher octane and pings a bit on 87.

How old are you guys? Leaded gas used to be 89 octane and old cars (like my '67 Ford Mustang) used it. They phased out the leaded gas (it was around 1984-85 in my state) but kept making the 89 octane for the older cars.

IIRC unless it’s a turbo then you should use 87

I have seen the manual of a Ford Windstar say that, while 87 is all you need, they do not recommend the use of 86 (all you can get in places like Albuquerque, Denver, and Salt Lake.) I think in Albuquerque it’s generally 86, 88, and 91. You can believe that that Windstar has never been filled with the 88. Otherwise, I second everything stent1st says. However, I have never seen a gallonage counter on top of the pump, but instead inside the handle guard–though I suppose that could depend on pump design.