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I've noticed some 35 owners (mostly on that other forum) have started putting a lower octane fuel in their FX in an effort to save some money at the pump. Because no one reported on the MPG change, I decided to try it myself.

Historically, I have always put 91 octane in my FX (highest we can get in CA) and I consistently average 15.7 MPG in roughly 80% city driving. I burned one tank of 89 octane and I've been careful not to change my driving patterns or style. Here are my results:

1. No knocking or pinging (as expected).
2. Acceleration is still good, but not as good as it was.
3. Sometimes I now smell a gasoline odor when I exit the car.
4. My economy has decreased from 15.7 to 14.8!

So here's the math:

With 89 octane it cost me $44 to fillup. With 91 octane it cost me $46 to fillup, however I can drive 18 miles further. To drive those 18 miles using 89 octane, it would cost me an additional $2.68. Therefore, driving with 91 octane saves me roughly $0.68 per fillup and I get to keep all the performance.

So if you are considering skimping a little at the pump, you better watch you MPG too. ::wink:
 

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I've actually been putting 89 into my 35 for about a month now, with no noticable decrease in performance. I think my MPG has gone up about .4.
 

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I have always used 91 and decided to try 89 on two tanks to have a good test.

On 91, I am getting 21.6 mpg average all the time.

but....

on 89, I dropped to 19.2 mpg :shock:

After that I ran that tank dry and filled with 91 and now back to over 21. Boy, I wont do that again.
 

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:D

I guess sometimes I do get some good charma.
 

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On short term you probably can run 89 octane. However, over a period of time I have seen injector problems in some vehicles due to running the wrong fuel rating. I dont know what specifically would cause this as the injectors only "inject" the fuel. I wonder if the MTB is to blame?
 

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OIIIIIIO said:
On short term you probably can run 89 octane. However, over a period of time I have seen injector problems in some vehicles due to running the wrong fuel rating. I dont know what specifically would cause this as the injectors only "inject" the fuel. I wonder if the MTB is to blame?
Correct me if I'm wrong, but I do believe the FX35 is capable of running indefinitely on 89 octane gas? Premium is only "recommended" and not required. At least that's what it says on the gas cap!
 

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OIIIIIIO said:
On short term you probably can run 89 octane. However, over a period of time I have seen injector problems in some vehicles due to running the wrong fuel rating. I dont know what specifically would cause this as the injectors only "inject" the fuel. I wonder if the MTB is to blame?
Most majors "save" their best detergents for Premium. For example note the current Shell advertisements. The base stock for ALL gasolines is the EXACT same on a regional basis (i.e. all gas in the metro-Houston area is derived from the same base stock). The only difference in brands is the Proprietary additives which consist mainly of detergents and octane boosters. Hence performance could differ due to differences in octane /performance boosters and fuel injectors could respond differently to different detergents.

However if people are having problems with injectors using a specific brand of gas then I'd switch brands and report the offending station. Octane rating should have NOTHING to do with injector efficiency; that is a detergent issue.
 

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ShadyJC said:
Correct me if I'm wrong, but I do believe the FX35 is capable of running indefinitely on 89 octane gas? Premium is only "recommended" and not required. At least that's what it says on the gas cap!
You are correct. Infiniti only recommends Premium in the FX35; however in the Owners Manual it does say that for peak/max performance a minimum of 91 octane is recommended.
 

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Discussion Starter #10
ShadyJC said:
I've actually been putting 89 into my 35 for about a month now, with no noticable decrease in performance. I think my MPG has gone up about .4.
That's great. It sounds like you are saving a few dollars at the pump. Generally it is a mistake to think that our MPG will change at all, however for us to see changes in opposite directions is very curious. Also, the weather has cooled and I'm not running the A/C anymore -- which suggest my MPG would even be worse with 89 had the climate remained the same.

I have also noticed that with 89 octane and moderate acceleration, the shifts happen at higher RPMs. This probably contributes to my poorer MPG because I'm applying more throttle to compensate for the loss of power.

Overall the car feels "heavier" to me and I'll never use lower octane fuels in this engine again.
 

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Another thing to consider is that lower octane gases can leave your pistons topped with carbon or the inability to help clean off carbon deposits that, over time, cause hot spots that lead to pre-ignition or detonation that causes pinging and engine wear. Rather than risk this, I am running premuim only in my baby.
 

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FXECUTIVE said:
Another thing to consider is that lower octane gases can leave your pistons topped with carbon or the inability to help clean off carbon deposits that, over time, cause hot spots that lead to pre-ignition or detonation that causes pinging and engine wear. Rather than risk this, I am running premuim only in my baby.
see... I knew I had fuel (pun) to my argument... :shock: :lol:
 

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I used 91 octane for maybe the first 5 fill ups. Then I read about FXers using 89 or lower. So, I started using 89 octane. I haven't noticed a difference in performance or mpg. I'm getting about 16mpg average (50/50 ratio freeway/city), but if I hop on the freeway for maybe 20 miles, my mpg jumps up to 18mpg. Then it slowly comes back to around 16mpg when I get back to my daily work route.

I like running the FX around the 3.5k-4.5k range. It seems to have much more pick up around there. This could attribute to my lower than expected mpg.
 

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Generally, I use 91 (our high grade), but as prices peak up at 2.30, I switch to low grade 87.

I keep a spreadsheet :read2: on every fill up .. My average is 19.3 .. for the two tanks of 87, I got 18.5 and 19.5. Sooo .. I'd have to say I didn't notice any real MPG change. -- And that's with 87 .. most of the above posts spoke of using 89.
 

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I certainly do not want *this* post to turn into a debate, but that possibility isn't going to stop me from posting. :)

most people do not understand octane. In it's simplest terms, the octane number determines the amount of anti-knock a fuel contains. The more octane a fuel has, the lower its flashpoint is.

What's this mean? This means that it takes more energy (read compression) to ignite the fuel. This means that the chances of the fuel pre-igniting (pinging/knocking) under the compression stroke is less.

For your typical car, you will generate the best performance running the lowest octane you can without producing knock.

It is true that in modern cars, the ECU can "maximize"/decrease performance based on the fuel you burn, but in reality each car can be quite different. Some FXs may actually ping with 89 while some do not. Many things can affect this. Over sensitive knock sensors can make the computer *think* the engine is pinging. Carbon build up (hot spots). Something as little as a metal burr in the combustion chamber glowing hot can pre-ignite the fuel in one cylinder causing decreased performance.

My wife and I run and have run '89 octane for most of the 31k miles we've put on the our FX45 in the past year. There have been no ill effects to date. I do not notice a difference between 89 octane and 92 octane in our FX45. That does not mean there isn't one. I have not done any conclusive testing, and in reality "driving" tests to measure MPG are not conclusive tests. There's just way too many variables involved. We average just shy of 18MPG with our FX45 AWD.

One good way to test would be to monitor the knock the ECU hears. If you can find a way to do this, then you can see whether the 89 you put in is knocking or not. If the ECU is not hearing any knock when your running 89 octane, then your getting maximum performance the ECU can allow.

Now that i've spouted off I just want to say I'm not an expert I don't get paid to know the above things. These are just things I've picked up modifiying my cars the past. So take what I said with that in mind.

For more information on Octane, a really good site to learn more would be http://auto.howstuffworks.com/question90.htm
 

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you people must be driving like grandma if your getting ~19 or more... I cant get above 16.5
 

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I get anywhere from 17 - 18.5 with a few bursts of spirited driving included. My wife does not appreciate the spitited driving or I bet I would be closer to 15. She gets sick when I take agressive turns and use hard accelerations. I try to justify that the FX is meant for this and we are mistrating it by driving it like a grandma. However, I have yet to win an argument with her so slow driving is it while she rides. :(
 

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Sorry, but I'm arriving late to this ocatane/mpg performance party. I haven't noticed a big difference in mpg with either 89 or 91. I have noticed that driving the FX35 for a few thousand miles in the past few weeks that the mpg has improved. I guess the original owner never got it out much.

Anyway, I'm still pissed that we can only get a max of 91 here in Cali. (except at the race track). Why is that!?
When I lived in Okla. we had a similar octane puzzle - in the western half of the state there was only 91 but in the eastern half there was 93.

I think you're right-on about the increased throttle required with 89. The computer is going to adjust the ignition timing up or down for the ocatne used. So, if you need higher-performance - go for the highest rating you can find/afford/justify.

We've been getting 15 to 16 in town and 20 on the highway. This is with a light foot and cruise control when possible (and I run with the a/c on pretty-wuch all the time). But man is it fun to floor it and run-off and hide when I have one of those drivers on my tail that tries to use me as a drafting partner!

Just .02 more cents

z
 
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