Continued: Gas Mileage

This is a discussion on Continued: Gas Mileage within the Off Topic & Humor Discussion forums, part of the The Back Porch category; Ok, few weeks ago I posted where a friend of mine said he got better mileage in his truck when he went to premium grade ...

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Thread: Continued: Gas Mileage

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    Distinguished Member Array ArmyCop's Avatar
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    Continued: Gas Mileage

    Ok, few weeks ago I posted where a friend of mine said he got better mileage in his truck when he went to premium grade gas and suggested I try same to see if I get better.

    I have a 4 cyl Jeep Wrangler. First couple tanks with regular I was avg'ing 17 MPG.

    First tank (partial) of MidGrade I got 17 point something - no noticeable difference.

    Second tank (total) of MidGrade I actually got 19.10 MPG. Depending on cost of gas (my stations price today) that was 1 to 2 cents better per mile. Still no "big" whoop but still surprised me nonetheless.

    I might just start running MidGrade all the time now.

    Before anyone asks if I drove any different tank to tank - no, same driving each tank, back and forth to work, Church and Walmart each tank....

    Not saying this would be the case for anyone else - several in another post said made no difference for them but if you haven't tried it you might just to see for yourself.
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    VIP Member Array jwhite75's Avatar
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    I saw a report on a national news program that said the Oil companies make more profit from "mid grade " than any other fuel they sell. The Octane difference is nominal between low and high test, and like your experiment concluded minimal result for mileage and the like. Plus start running mid or Premium for an extended time then switch back to regular because of the prices that are going to go up major again...and listen to your motor. It will start pinging like its going to throw a rod through the block. I cant afford to run premium or mid grade, The only added expense I go to is I have ran Mobil 1 Synthetic motor oil since brand new and every 2 oil changes I run a Fuel ssytem cleaner through a tank of fuel. My mechanic say my motor is one of the cleanest internally he has seen for over 100,000 miles, it has 159,000 and is running like a champ.
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    Distinguished Member Array GunGeezer's Avatar
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    In theory, mid-grade and premium gas contain additives that make the gas less volatile, that is burn slower. Higher compression engines develop more heat from compressing the fuel/air mix, and without the additives found in premium blends, the fuel would simply explode from the heat of compression rather than the spark. That's what produces the signature knock or ping. Also, the fuel/air mix would truly explode rather than a controlled burn that is the design feature of a gas engine. So, in basic theory one could expect a tiny increase using higher grade gasoline in engines not requiring it. In reality, it is insignificant and merely a waste of money. Also, computerized engines can actually sense the octane rating of the fuel and alter ignition timing and fuel delivery to compensate for fuel differences; vehicles that can burn gas or ethanol are an example of this technology taken to the extreme. A factor most people don't consider is the psychological expectation of getting better mileage with a supposedly fuel saving additive or device. Thus, they unwittingly alter their driving habits, at least in the short run, and do indeed appear to get better mileage. In time they go back to their old ways and their gains disappear. Smart money says, don't use any higher grade fuel than your vehicle requires.

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    Distinguished Member Array C9H13NO3's Avatar
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    I use standard grade gas in my truck. I only use premium in my motorcycle because it has almost a 16:1 compression ratio.
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    Senior Member Array Sig35seven's Avatar
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    Your fooling yourself and wasting money at the same time. There is no way you could have a huge percentage gain in economy like that. IMPOSSIBLE.

    The answer you seek should come from controlled scientific tests done on a dyno in controlled temperature and atmospheric pressure. Scientific tests have shown what you are suggesting is not correct. Not even close.

    The biggest influence on fuel economy is your right foot.

    BTW... have you checked your air pressure in your tires lately? It has a much bigger influence on gas mileage than would the additional cost of a middle grade fuel and it's FREE!
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    Distinguished Member Array ArmyCop's Avatar
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    Sig35seven: You sound like me talking to my friend when he first suggested I do this & how he was getting better on Premium - But - I don't see how I could be fooling myself when the figures were as follows:

    Regular tank - 227.2 miles on 13.172 gals = do the math and it's 17.24 MPG

    MidGrade tank - 277 miles on 14.500 gals = and the math shows it to be 19.10 MPG
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    Quote Originally Posted by ArmyCop View Post
    Sig35seven: You sound like me talking to my friend when he first suggested I do this & how he was getting better on Premium - But - I don't see how I could be fooling myself when the figures were as follows:

    Regular tank - 227.2 miles on 13.172 gals = do the math and it's 17.24 MPG

    MidGrade tank - 277 miles on 14.500 gals = and the math shows it to be 19.10 MPG
    If I were you, I'd try it averaged over several tanks of gas. Gas pumps aren't terribly consistant on what point they shut off when filling, plus it will compensate for minor differences in driving between tanks.

    However, since I'm not you, I won't bother to even try, since I drive a Diesel.

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    Senior Member Array Sig35seven's Avatar
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    Quote Originally Posted by ArmyCop View Post
    Sig35seven: You sound like me talking to my friend when he first suggested I do this & how he was getting better on Premium - But - I don't see how I could be fooling myself when the figures were as follows:

    Regular tank - 227.2 miles on 13.172 gals = do the math and it's 17.24 MPG

    MidGrade tank - 277 miles on 14.500 gals = and the math shows it to be 19.10 MPG
    Here is a different test for you to perform. Put a different driver behind the wheel and don't tell them what's in the tank. Also, they must drive on the same roads with same conditions.

    I did this with a friend who claimed that acetone increased his mileage. He said... "I got 28 in the city and about 32 on the highway. With acetone I get 34 in the city and 40+ on the highway."

    He agreed to let me control when acetone was put in his tank. The result was NO increase at all. I believe subconsciously he drove more conservatively when trying to achieve the best mileage and I believe you are doing the same.

    I did this test with my wife driving my Caddilac. North star V8. To and from work everyday the same route. Switched from regular to premium with NO improved results. This is EXACTLY the same results that the controlled scientific tests have shown.

    It is however good to see you are driving with a lighter right foot and may have improved your driving style. Keep up the good work.

    Just for fun... since you're trying to get improved mileage...tell me what your tire pressures are??? When was the last time you checked them? Do you own a tire gauge?
    "Confidence is food for the wise man but liquor for the fool"

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    I'm a numbers guy - it's the engineer gene I carry. Also a gearhead.

    I record miles and gallons for every car I've owned since nearly forever. It takes a LOT to generate a 10% increase in gas mileage.

    Depending on a lot of variables - including the specific fuel injection system, compression ratio, wide-band or narrow-band oxygen sensor, and ambient air temperature, to name a few - using higher octane fuel can improve gas mileage by maybe 2-3%. The problem is, as Sig35 suggests, there are so many other factors involved that an actual fuel economy improvement can be buried by the "noise" of the other variables. ArmyCop, I won't dispute your measurements, but I will say that you need to look at a long-term average and not just a couple of tanks full.

    Being partial to German and Scandinavian cars, and having commuted 75+ miles daily for 15 years, what I have seen over the years is that ambient temperature has more of an effect on fuel economy than almost anything else except driving habits and tire pressure. But even distilling that out when the EPA mandates seasonal changes in gasoline formulation takes a lot of data gathered over a long time.

    If your car requires premium fuel - and some do (e.g., a lot of turbocharged and high-output European cars), you will give up some performance and/or some fuel economy by using regular fuel. Will you notice it in one or two tankfuls? No, because the engine control system is adaptive - it "learns" your driving habits and its octane diet over time. You would have to re-set the engine control computer at each fill-up to see a change, which is inconvenient but not difficult. You would notice the difference in performance at the drag strip, though.

    If your car does not require premium fuel, then unless your engine is modified from stock to take advantage of the higher octane, you won't see improved fuel economy or performance.
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    Now that car engines are computer controlled you shouldn't switch. Your car was dialed in for low grade and when you switched it may have fooled your engine. Your engine will eventually compensate and your mileage will fall.
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    Quote Originally Posted by atctimmy View Post
    Now that car engines are computer controlled you shouldn't switch. Your car was dialed in for low grade and when you switched it may have fooled your engine. Your engine will eventually compensate and your mileage will fall.
    Not so. Virtually all cars sold in the US since the OBD II regs went into effect use adaptive logic in the engine control computers. Short of disconnecting sensors or simliar physical changes, you can't really "fool" your engine.
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    Quote Originally Posted by gasmitty View Post
    Not so. Virtually all cars sold in the US since the OBD II regs went into effect use adaptive logic in the engine control computers. Short of disconnecting sensors or simliar physical changes, you can't really "fool" your engine.
    Call it what you will but my owners manual for my 2001 Cherokee tells me specifically not to change octane levels because the motor will tune itself to what gas I regularly use.

    My assumption is that upgrading octane will cause an increase in performance for a short time (a tank or two). Then as the engine re-tunes to the new octane level performance will return to normal.
    Mark Twain:
    The government is merely a servant -- merely a temporary servant; it cannot be its prerogative to determine what is right and what is wrong, and decide who is a
    patriot and who isn't. Its function is to obey orders, not originate them.

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    Senior Member Array Sig35seven's Avatar
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    The only thing the computer would control or 'change' due to higher octane would be through the knock sensor(provided it's a newer vehicle and has one). In a higher compression engine that is run on regular, 'lower octane fuel' the knock sensor would retard the timing creating a lower cylinder pressure and allow for the use of a lower octane. There could be a performance increase due to more advanced timing causing a higher cylinder pressure when switching back to higher octane when the engine is placed under a load. Again we're talking about a high compression engine. I don't think his Jeep would qualify as high compression. Under normal driving you would see NO improvement in gas mileage.
    "Confidence is food for the wise man but liquor for the fool"

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    Distinguished Member Array ArmyCop's Avatar
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    Well, Since my first several tanks with Regular consistantly got 17 or 17.something before I even considered switching - I'll keep using MidGrade and keep checking MPG every tank and see if it goes back down to 17 or so.

    That should tell me something.

    Also - as suggested - sometime in the future I'll swap with wife and let her have the Jeep for a couple weeks (not tell her what I'm up too) and do the reg/midgrade calculations and see what we get.

    Of course I'll post results no matter what the findings turn out to be.

    It'll be a couple months before I repost on my findings.

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    Quote Originally Posted by Sig35seven View Post
    Your fooling yourself and wasting money at the same time. There is no way you could have a huge percentage gain in economy like that. IMPOSSIBLE.

    The answer you seek should come from controlled scientific tests done on a dyno in controlled temperature and atmospheric pressure. Scientific tests have shown what you are suggesting is not correct. Not even close.

    The biggest influence on fuel economy is your right foot.

    BTW... have you checked your air pressure in your tires lately? It has a much bigger influence on gas mileage than would the additional cost of a middle grade fuel and it's FREE!
    Exactly. Unless you use a dyno there are WAY too many variables. Traffic, speed, temp, wind, type of roads, and right foot.

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