308 Gas Mileage Hi-test vs reg. unleaded

Discussion in 'Technical Q&A' started by ClydeM, Aug 4, 2005.

  1. ClydeM

    ClydeM F1 Veteran
    Rossa Subscribed Owner

    Nov 4, 2003
    Wayne, NJ
    Full Name:
    Clyde E. McMurdy
    for kicks, I'd thought I'd see if I had any difference in mileage from hi-test (92,93 octane) to regular unleaded (87 oct) on my 308. (Hess brand)

    15.89 mpg hi-test (about .20 more a gallon over regular)
    15.39 mpg regular unleaded.

    This is over 4 tank fill-ups.
    I noticed no additional rough engine, pinging, hesitation, etc.

    Thought I'd share my findings. I'll keep going with reg. unleaded for a while for a better sampling.

    I have found differences among vendors which I noted in another thread. That's why I stuck with Hess which I found as the best performer (based on mpg again) in my area. No, we don't have Chevron in my area ;)
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  3. hardtop

    hardtop F1 World Champ

    Jan 31, 2002
    Full Name:
    There have been some controlled tests that show cars get better mileage with regular (assuming that is what they were designed for). Regular actually has more stored energy per gallon, so this makes sense. However, the differences are always tiny.

  4. djs308

    djs308 Formula Junior
    Silver Subscribed

    Sep 2, 2002
    Long Island, NY
    Full Name:
    David S.
    I've often wondered about this myself (the effects of using a lower grade gas). But now with gas prices in the stratosphere, you can save a few bucks on a fillup with "regular". Keep us posted on your results.
  5. loki

    loki Guest

    Jul 25, 2005
    If your car runs on a lower octane then use it. The definition of octane is a resistance to burn. Hirer octane fuels do not burn as easily and prevent pre-detonation. If you don't have to use it, it is a waste of money.
  6. ham308

    ham308 Formula Junior

    Nov 3, 2003
    NE Switzerland
    Full Name:
    Richard Ham
    If your gas prices are in the stratosphere, that must put our European $7/gallon somewhere between Mars and Pluto ?! :)
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  8. Mike328

    Mike328 F1 Rookie
    Rossa Subscribed

    Oct 19, 2002
    Boulder, CO
    Full Name:
    Hi Clyde, thanks very much for posting this data... Always interesting to see.

    Unfortunately, as a guy with a "stats" background, I don't believe that your sample size--i.e. the number of gallons of gas you've burned--is really big enough to conclude for sure that one is better than the other, with such a small difference. In other words, 1/2gal better (about 3%) could (I think, need to do the math to know for sure) be due strictly to "chance" or "sampling error" instead of an actual underlying improvement in mileage. As the difference goes up, or the number of gallons you burn goes up, this is less likely.

    I'm grateful that you've posted, because I myself am interested in fuel consumption. Check out this thread:

    My 328 Fuel Log After One Year of Ownership

    ...BUT, I don't think we can really conclude anything from the data you have so far on mileage vs. octane... Would have to do the math to know for sure.

  9. don_xvi

    don_xvi F1 Rookie

    Nov 1, 2003
    Outside Detroit
    Full Name:
    Don the 16th
    As an engineer that's done lots of fuel economy testing, on-road and in the lab, it'd be hard to argue that it's scientifically valid, but interesting info. We'll skip all the combustion theory for now... ;)

    P.S.-just an interesting side note, knock sensor cars are REQUIRED by law to get within 3% of the same fuel economy with regular vs. premium.
  10. WAR EGL

    WAR EGL Formula Junior

    May 8, 2003
    SE Connecticut
    Full Name:
    Additionally, with the mpg ClydeM has listed, its essentially even money whichever he chooses.
  11. ClydeM

    ClydeM F1 Veteran
    Rossa Subscribed Owner

    Nov 4, 2003
    Wayne, NJ
    Full Name:
    Clyde E. McMurdy
    {head hanging low} I'm a stats guy too. You are absolutely right, my sample size is not even close to significant. brand, weather, driving conditions, tune-ups etc comes into play. But I've tracked my mileage for 5 years so I'm comfortable that with further monitoring the results will be similiar for regular vs unleaded. My only concern is any additives that might be found in Super vs regular that might clog injectores or such. So I'll be adding a container of techron every 3K miles or so. stay tuned
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  13. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    North shore, MA
    Full Name:
    THE Birdman
    I'm not a stats guy (one course in college and I have forgotten 95% of it). While we can say that the two decimal points of precision are probably not accurate, I think his comparison reasonably concludes that there isn't much difference between the two--certainly not enough to choose one or the other based on fuel economy.

    I put regular 87 octane in my 308 and Mondial. Have never noticed a difference in economy or performance versus 89 octane. I have never bothered to even try 93 octane.

    I really feel for you guys in Europe with fuel prices. I was watching Top Gear a week or so ago and they mentioned that the price of a gallon of gasoline in England was about $8 right now. Holy crap! We should quit our complaining! It takes dedication to drive a car that gets 15 MPG at $8 a gallon!

  14. Iain

    Iain F1 Rookie

    Jan 21, 2005
    It's not quite that bad. Its around 93p a litre for Shell Optimax (98 RON) which converts to around $5.90 per US Gallon (at an exchange rate of $1.75:£1).
  15. Dr Tommy Cosgrove

    Dr Tommy Cosgrove Three Time F1 World Champ
    Rossa Subscribed Owner

    May 4, 2001
    Birmingham, AL
    Full Name:
    I have always used the higher octane fuel in my 308 ONLY for preignition damage insurance. That's all. I don't know if it does, will, or has ever pinged back there ( I have a Tubi - it's too loud to tell) but I live in Alabama. We have hot summers. My QV runs hot, I use the air conditioner, and I live on top of a big hill with traffic. I would rather waste my money on expensive gas then an engine rebuild. Besides, if my major concern was saving $3.50 at the pump per fill-up, I wouldn't be driving this car. I would have a Toyota.
  16. LSU348

    LSU348 Formula 3

    Dec 19, 2003
    Sugar Land
    Full Name:
    I have got to go make another long run and recheck my mileage per gallon (85 QV - GTB). I swear I was over 20...I must have messed up. It was a full tank run on a rally with the local FCA and included top speeds and put put driving from traffic light to traffic light. Just about every post I see says I should see 16-18 mpg at best. I am not running lean (but pass emmissions without any tweaking).

  17. DN35

    DN35 Formula Junior

    Nov 22, 2003
    Full Name:
    D. Norton
    I've always used 87 octane regular unleaded in my '78 308 and have never had any pinging, etc. Believe me, if I heard the slightest ping the next nozzle in the tank would be the fancy one. I'm also of the belief that regular burns hotter and quicker and would keep my engine more free of gunk than Premium.
  18. ClydeM

    ClydeM F1 Veteran
    Rossa Subscribed Owner

    Nov 4, 2003
    Wayne, NJ
    Full Name:
    Clyde E. McMurdy
    agreed. But money is not the major concern. I found it interesting: I too subscribed to the dogma of Hi-Test gasoline for performance. It appears to be an erroneous assumption. I'd also like to know, for sure, if addititives play into engine cleanliness or if that's marketing hype. Yes, I've seen the countless threads here on that very subject.

    after tracking brands for quite a while, I have noticed a difference in mpg between brands. Presumably, better mileage is also an indicator of better performance and cleaner engines and happy smiles all the way around. :)
  19. ClydeM

    ClydeM F1 Veteran
    Rossa Subscribed Owner

    Nov 4, 2003
    Wayne, NJ
    Full Name:
    Clyde E. McMurdy
    Ok, here it is year end and I've used nothing by regular since that post.
    I've got 3673 miles using 232 gallons with 28 fills.
    avg MPG for regular: 15.83mpg. (predominately Hess brand)

    So in my book, no performance difference between Hi-test & unleaded. The station I use is very high volume so stale gas isn't a problem. Then it's just a matter of additives between regular & hi-test.

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