MPG difference between 87 octane and 89 octane?

Discussion in 'Fuel Economy' started by diamondlarry, May 24, 2012.

  1. diamondlarry

    diamondlarry Super MPG Man/god :D

    I hope I can make the reason for asking my question clear so here goes:

    The manager at the station I usually go to told me a couple of years ago that their fuel doesn't contain ethanol. I recently saw a tanker there that I don't remember seeing deliver fuel there before. This makes me wonder if the non-ethanol status has changed. I do know that there is a fuel distributor in the same town that does sell non-ethanol retail at their depot but, it's 89 octane.

    Now, here is where I hope I can make myself clear enough: I fully realize that the octane rating is a measure of how much the fuel resists premature detonation and has no bearing on overall energy content. So my thinking is that the lower the octane rating, the easier the fuel will ignite thereby insuring it would all burn quickly and completely. So, since an Insight is so very sensitive to changes (mpg is already so high that that even a small % shows up as a huge mpg drop), would there be any mpg penalty from going to 89 octane? I do know there would be a $ penalty but I'm wondering about the mpg penalty. I know the owner of the distribution company so the price may not really be an issue anyway.
  2. abcdpeterson

    abcdpeterson Well-Known Member

    I have tried a few times and in my experience higher octane fuel in a engine that doesn't require it will do nothing for you. Nothing good, nothing bad, just nothing.

    if the price is not an issue run the one that does not have ethanol.

    I have had only 1 car (Chevy LUV) that wanted High octane. It took a big power hit if I ran regular gas.
  3. RedylC94

    RedylC94 Well-Known Member

    I don't believe you'd experience any penalty going from 87 to 89 octane, assuming both are E0. On SOME cars there'd be a mpg penalty going the other way (if ignition timing retarded automatically to eliminate knock).

    CPLTECH Well-Known Member

    My sister has a recent PT Cruiser and was complaining about the mileage for such a small car. She came for a visit a few months ago and I put some MOTORKOTE (now also available at wallyworld) oil additive and said the MPG kept increasing on the way home (100 mile trip).

    Last week she noticed a big bump in MPG by using 89 vs. the 87 octane. I, too, find an 8% increase in my Pathfinder when I use Prem. So that justifies the additional cost.

    Some say the extra octane doesn’t help them. That may be true in their case. You just have to experiment for yourself to find the answer. Seems the answer is vehicle dependent.
  5. herm

    herm Well-Known Member

    You want a cheap octane boost?.. mix in about 10% E85 (or 20% if its pure gas) with your regular gasoline.. give it a couple of weeks for the timing to self-adjust and see what mileage you get. You might be pleasantly surprised.
  6. 08EscapeHybrid

    08EscapeHybrid Moderator

    Yes, when I used to work for a fuel station, whenever I ordered "midgrade" the bill of lading would show 1,000 gallons regular unleaded, and 200 gallons premium unleaded.
  7. ALS

    ALS Super Moderator Staff Member

    My Volvo requires at least 91 octane. I made the mistake of getting cheap and filling with 89 octane on a trip to Florida back in 2005 or 2006. The next fuel stop I was quickly back to premium. I can't remember exactly how bad the fuel mileage hit was. I probably got no more than 22 or 23 mpgs because I didn't even get 500 miles out of a 21 gallon tank. It was substantial enough that I have never filled up again with anything lower than 91 octane.

    This was before E10 was rampant across the U.S. With straight gas I would get in the mid 29's at 65 mph.

    One of these days I'll have to fill the Prius with 89 when I plan on a short road trip and see if it helps the fuel mileage.
  8. 08EscapeHybrid

    08EscapeHybrid Moderator

    If the Prius has an Atkinson cycle engine like my FEH does (I suspect so) then all things being equal, the lower octane fuel will perform better. That being said, if I had to choose between E10 87 and E0 93, I'm buying the 93. The ethanol content will effect the mileage more than the octane.
  9. ALS

    ALS Super Moderator Staff Member

    I lost five miles per gallon in the Prius when I was forced to run E-10 over E-0. :mad:
  10. 08EscapeHybrid

    08EscapeHybrid Moderator

    Same here. I lose 5-8 MPG when forced to use E10 in the FEH.
  11. herm

    herm Well-Known Member

    I cant understand what is going on with these reports of big mpg losses... the energy difference is not that high, and the octane boost of E10 is not going to reduce the mileage.

    There must be something else in the mixture besides ethanol that has changed... perhaps the timing on a Prius or FEH does not self adjust but I doubt it.
  12. 08EscapeHybrid

    08EscapeHybrid Moderator

    There's a noticeable difference in all of my vehicles, except my motorcycle. I haven't spotted a trend with ethanol free fuel in that vehicle, but then again, ethanol free premium is hard to come by, and I can rarely run it more than 1 tank. In my 98 GMC K1500, its a 2 MPG difference, fully loaded @ 70 MPH. In the FEH, I'll do 27-31 MPG on E10, and 30-36 MPG on ethanol free. My last tank in the FEH was 36.12 MPG (371.5 miles/10.284 gallons), and I had a fair amount of a/c use on that tank as well.
  13. RedylC94

    RedylC94 Well-Known Member

    Maybe you're looking for an engineering explanation for a psychological phenomenon.
  14. some_other_dave

    some_other_dave Well-Known Member

    In most stations, mid-grade is created by mixing the high-octane and regular-grade gas at the pump. The station only stores two grades of fuel, and they make the middle stuff by giving you a 50-50 mix of the two.

    Some cars are set up to take advantage of higher-grade fuel than the minimum they require. Others are not. The specifics of the engine design and engine management system design generally determine that. However, there seem to be cases of individual cars reacting differently to changes in octane, even though they were the same model of car! So individual testing would be necessary to determine the effect in your case.

    Ethanol has less energy content than gasoline. It's on the order of 30% less, which is one of the reasons that the "E10 Handicap" for our quarterly FE challenges is 3%. In theory, 10% ethanol will decrease your FE by about 3%. In practice, it is often different. Some cars appear to really be built for E0, and suffer a larger than 3% hit. Others don't seem to care one way or another. I wouldn't be surprised if there were a few that worked better with E10--but I wouldn't be surprised if there weren't, either. Again, testing of your particular car will help you determine how it reacts to fuel with ethanol in it.

  15. shadescape

    shadescape Banned

    can you provide any proof to this statement
    I managed a gas station some years ago
  16. 08EscapeHybrid

    08EscapeHybrid Moderator

    I've worked in a couple of fuel stations, and newer "blender" pumps do that. Older stations with a separate tank for midgrade have it mixed when its loaded on the truck. Midgrade fuel is not refined, its mixed. The refineries only make regular and premium. Anything in the middle is a mix. When I worked for a Gov't run fuel station on a military base, when we ordered midgrade, the order was 1,000 gallons regular, and 200 gallons premium.

  17. shadescape

    shadescape Banned

    thanks for that info- never would have guessed
  18. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    some other dave , I understand that your MINI asks for premium fuel. Have you tried different octanes ? What were your results ?
  19. some_other_dave

    some_other_dave Well-Known Member

    I have never used anything but CA-spec 91 octane premium fuel in my MINI. I will continue to use it as long as the car is under warranty, just in case.

    The owner's manual says that 89 octane fuel is acceptable, but recommends that I use 91.

  20. chilimac02

    chilimac02 Bible Professor & Minister

    2-3mpg drop in my old accord from using e10. But now you cant buy e0 within 50mi of here.

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