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Cadillac STS Forum - 2005 through 2012 Discussion, Gas mileage in Past Cadillac Vehicle Discussion; I don't have a Phd in physics! That's why I would like to find out how you know you get ...
  1. #31
    malatu's Avatar
    malatu is offline Cadillac Owners Connoisseur
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    Re: Gas mileage

    I don't have a Phd in physics! That's why I would like to find out how you know you get better gas mileage at 90 than you do at 65 mph. What method did you use to determine this? Maybe my car gets better mileage at that 90 mph too! Please share.

    Quote Originally Posted by atlrus View Post
    No, it does not. I love how everyone all of a sudden has a PhD in physics

    Ever thought that my STS maybe doesn't get as good mileage at 70mph as it should?!?
    There are many possible explanations as to why I get better mileage at 85 rather than 70. I never claimed that anyone else would, as well, although someone else earlier noted they've seen the same thing with their sts.



    I couldn't honestly care less whether you are skeptical or not - It has zero effect on my gas bill I didn't post my experience here to seek validation, just to share it with the OP.

  2. #32
    btlegacy is offline Cadillac Owners Member
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    Re: Gas mileage

    I recently wrote an article for an online publication and figured it was appropriate to share for this topic.

    There are many stories and anecdotes and what fuel octane is best and what effects it has on a vehicle. One of the most common discussions is whether higher octane will give you a benefit. There are many myths about fuel octane and power and gas mileage and I will try to break them down using real world data.

    Octane and Knock


    The Truth About Gas! Does Premium Fuel Perform Better?Expand

    The octane of a specific fuel is a number that states how likely the gas is to ignite under pressure. The scale works so that the higher the number, the lower the chance of igniting under pressure and causing detonation inside an engine. The marketers that sell gas would love to lead you to believe the higher numbers are cleaner or more efficient but that is simply not the case unless the higher octane is mixed with special detergents or other additives (Thanks for the tip ddavidn).

    . Higher octane may be slightly more efficient when combined with modern engine computers but it does not save money overall.

    The main question people ask in regards to this explanation of octane is "Why wouldn't I want the fuel to ignite in an easier fashion? "The answer for that is that, higher octane provides for less of a chance of pre-ignition of fuel which causes knock. In a normal scenario, fuel is sprayed into an engine and only ignites when the spark plug lights up and therefore causes proper movement for the engine. In a pre-ignition or knock condition, the fuel ignites before a spark plug lights up and causes the engine to run less efficiently or to get damaged.

    Premium Fuel Required

    The Truth About Gas! Does Premium Fuel Perform Better?Expand

    In most modern engines, the combination of low compression ratio and advanced engine management prevents knock from happening. The compression ratio is the pressure that air and fuel is put under inside a cylinder. Engine management works by monitoring knock sensors to see if there are pings and adjusts fuel and timing accordingly to manage it. In this case, if the vehicle requires 87 Octane then the engine computer has been tuned for that specific octane and will run efficiently with it.

    Many of the modern high compression engines or engine that use forced induction require premium fuel at 91 octane or higher. The reason for this is as the compression ratio of an engine increases the chance of knock increases as well so higher octane prevents the knock from happening. In this case, putting a lower grade of gas will cause knock to become apparent and the engine computer will make changes to prevent the level of knock.In testing, we have seen lower octane to cause efficiency to go down as much as 20%.

    Cheaping Out but Losing Money


    The Truth About Gas! Does Premium Fuel Perform Better?

    To put our theory into real world terms let's say "Car A" usually averages 25 mpg, requires premium gas and has a 10 gallon tank. The price for 93 Octane is $3.50 and the price for 87 Octane is $3.10 for our example. A full tank of 93 Octane will cost $35.00 while a tank of 87 Octane will cost $31.00.Driving on the 93 Octane yields 25 mpg so the driver is able to pass 250 miles for $35.00 dollars and the cost per mile will $0.14. With the 87 Octane gas in the tank the engine computer scales back and the car only averages 20 mpg. Driving on the 87 Octane yields 20 mpg so the driver is able to pass 200 miles for $31.00 dollars and the cost per mile will be $0.16.

    The example above is simplified but shows that even though the entry cost is lower and in the end, the gas will end up costing more. What's even more important is that the detonation caused by running lower octane will reduce the health of an engine over time and if strong enough can cause permanent engine failure at once.

    The Placebo Effect

    Many people live by the fact that even though their car requires 87 Octane, they choose to fill up with 91 or 93. There are a few reasons for this action and one of them is people stating that it keeps their engine clean. This is a myth as all of the fuels from a single source will run just as clean in any engine. The biggest statement that is put out is that running higher Octane will cause the car to run more efficiently. We did some testing and found that in certain situations, the engine computer will adjust and run in a more aggressive manner but we never saw increases in fuel efficiency of more than 3%.

    Using an example similar to the one above, we will show the economics of these changes. Let's say "Car B" usually averages 30 mpg, requires 87 Octane and has a 10 gallon tank. The price for 93 Octane is $3.50 and the price for 87 Octane is $3.10 for our example. A full tank of 93 Octane will cost $35.00 while a tank of 87 Octane will cost $31.00. Driving on the 87 Octane yields 30 mpg so the driver is able to pass 300 miles for $31.00 dollars and the cost per mile will $0.103. Driving on the 93 Octane yields 31 mpg so the driver is able to pass 310 miles for $35.00 dollars and the cost per mile will $0.113.

    The example above is simplified but shows that running higher octane in the best possible engine management scenario still yields a higher cost than running the lower octane. On the economics side of this argument, running the higher octane is a waste of money. On the engine health side, if a car has been designed and tuned for 87 Octane, it should not have any issues if it is properly maintained.

    In the end, the best advice is to run what you manufacturer recommends.

  3. #33
    MacMuse's Avatar
    MacMuse is offline Moderator
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    Doesn't anyone want to see the speeding tickets from trying to run a car at 90mph long enough to establish fuel usage characteristics? Are we sure we're not talking kph?

  4. #34
    atlrus is offline Cadillac Owners Fanatic
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    Re: Gas mileage

    Quote Originally Posted by MacMuse View Post
    Doesn't anyone want to see the speeding tickets from trying to run a car at 90mph long enough to establish fuel usage characteristics? Are we sure we're not talking kph?
    Lol, I actually always drive 85-90 on the highway, I don't see how people can drive this car at anything less, may as well buy a Camry, if you'd be driving at 70mph.

    Tbh, I don't see any other reason to own this car but the speed - it rides like a horse-drawn wagon, fit and finish, along with reliability is WELL below its European and Japanese competitors and you'd be much more comfortable cruising 70mph in a Buick, if you want to stay with US-badged car...

    As far as running it at 90mph long enough - I have taken it cross country twice and with a good detector and a couple of apps you should avoid tickets, no problem.

  5. #35
    KRSTS is offline Cadillac Owners Enthusiast
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    Re: Gas mileage

    You the man

  6. #36
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    C&C
    C&C is offline Cadillac Owners Connoisseur
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    Re: Gas mileage

    So first you start an outlandish boast about your mileage (doing 90), now the STS is sub-par (to its competitors), and "it rides like a horse-drawn wagon"; looks like you're 'trolling' for reactions. Other than to mention these two indiscretions, I'm done with this thread (at least trying to discuss opinions) with you. Have a nice day, sir. Fool me once, shame on you, fool me twice shame on me. No malice intended in my above comments (but happy to be done here).

  7. #37
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    This thread reminds me of a guy in my submarine "A school" who installed one of those tornado intake things in his Hyundai. Myself and the other gearhead in the class tried to explain to him the fact that he was NOT seeing the ridiculous gas mileage and power increase he claimed, but it was a waste of breath.
    Also, since when is a PhD a requisite to present a fact on a particular subject? Must I hold a PhD in Mathematics to tell my child that 1+1 is in fact 2 and not 3?

  8. #38
    Submariner409's Avatar
    Submariner409 is offline If it won't run, install a sound system
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    Re: Gas mileage

    "Gas mileage" is like whiskey, women, and guns - everyone has his own opinion and that won't change. "Miles per tank" is a completely phony benchmark. MPG/MPK is the worldwide standard. One of my trucks, with a 27 gallon tank, blows away my STS - with an 18 gallon tank.......... so the truck gets "better mileage" ?

    ........... and the STS on a Florida run, 65 - 75 mph for the Rt.95 Interstate, gets just over 25 mpg, so for a 16 gallon refill I get over 400 miles/tank...... 360 per fillup woulod be a ~3 mpg loss.

    It doesn't take a physics degree to figure out that, in a "modern" motor vehicle, the most economical road speed is between 55 and 70 mph. Even at 70, wind drag creeps in and more power is required to maintain speed ........... More power = greater throttle opening = less fuel mileage.

    Install a ScanGauge-II or an UltraGauge and watch power demand vs. throttle opening vs. speed. No way to argue with graphic numbers in dynamic time.

    Submariner 409 - 6 boats, COB in 2, 26 years.

  9. #39
    atlrus is offline Cadillac Owners Fanatic
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    Re: Gas mileage

    The PHD comment was not about actual physics, but the lack of logical thinking with the few of the posters.

    All I am saying is that my STS gets better gas mileage in the 85-90mph range than at 70mph. READ IT AGAIN SLOWLY! I've never claimed that 85mph is the best fuel-efficient speed of any STS. Read that one more time, as well. As I mentioned earlier, maybe something is wrong with the car and I don't get an optimum gas mileage at 70mph, as I should, which is why I say some people here fail to think logically and try to fit the problem around their answer... And what's my answer why the car gets better mileage at 85mph? I don't know, never claimed to do either. But to keep on harping that it's impossible, when I am the one that is driving the car and paying for the gas is nothing short of acting childish.

    Are these scientific measurements? Of course not, it's personal observation from the 20K miles I've put on this car actually driving it in pretty much all speeds and road conditions.

    "Miles per tank" is a completely phony benchmark. MPG/MPK is the worldwide standard. One of my trucks, with a 27 gallon tank, blows away my STS - with an 18 gallon tank.......... so the truck gets "better mileage" ?
    I think you missed the point. Everyone who has been a driver for a while has a pretty good idea about how much gas their car burns and everyone has an idea about "a tank of gas", for most of us being around 15gals in these cars.
    If you want to be precise, the proverbial MPG you mentioned as a standard is just as phony - that's why there is no set MPG, but a bunch of BS like "city" "highway""combined" etc., not to mention factors like "uphill" "downhill" "offroad" etc. Put all these things together and your "standard" MPG is just as phony as our regular joes "tank of gas"

    -----------

    looks like you're 'trolling' for reactions.
    Your age excuses you from not understanding the meaning of "trolling".

    I'm done with this thread (at least trying to discuss opinions) with you. Have a nice day, sir.
    Please, note, your opinion was never solicited by me, nor will it be. Next time do like I do - just give your opinions to the Original Poster, instead of trying to start a flame war with someone else in the middle of the thread.
    Have a nice day, as well. And don't forget to stay in the right lane when you see my headlights coming

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