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Cadillac DTS Forum - 2006 through 2012 Discussion, Fantastic gas mileage on highway trip - 2008 DTS in Past Cadillac Vehicle Discussion; Originally Posted by MoistCabbage The MPG readout on the DIC uses the exact distance driven, and the exact amount of ...
  1. #16
    rebel7254 is offline Cadillac Owners Member
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    Re: Fantastic gas mileage on highway trip - 2008 DTS

    Quote Originally Posted by MoistCabbage View Post
    The MPG readout on the DIC uses the exact distance driven, and the exact amount of fuel that is injected to reach its number, THAT is accurate.
    Why would the MPG on the DIC be accurate, but not the gallons used? The computer has to know the fuel usage to figure out mpg. Why would it be using a different counter for gallons consumed than what it shows on the DIC? And why would it be using a different distance counter than what shows on the DIC for Trip A if I reset it at fill-up?

    As for questioning Google maps, I just loaded the same route in MapQuest and it came up with 445 miles.

    Unfortunately by now I can't remember exactly what my Trip A mileage was when I reached my house. I do know that it was in excess of 450 when I filled up the next day though.

    Like I said, I'm not going to get bent out of shape over some people on a forum not believing me. The main goal of this thread was just to share my experience with everyone else, and if folks think I'm making it up, I couldn't care less.

  2. #17
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    Not trying to get anyone bent out of shape. My point is, if you want dead accurate MPG for any given trip, reset the AVG MPG readout right before you start driving, and look at it as soon as you arrive at your destination.

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    Re: Fantastic gas mileage on highway trip - 2008 DTS

    I just went out and checked my GALLONS USED...
    It is NOT in WHOLE gallons...
    It shows 68.4 since the last time it was reset...

  4. #19
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    Post edited, but the AVG MPG readout is still the most accurate way to calculate average.
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    Re: Fantastic gas mileage on highway trip - 2008 DTS

    I'm looking at my receipt right now from where I filled up in the town I was leaving. I put 12.61 gallons in the tank. According to Google maps, the route I took home was 438 miles. I did not have to stop to fill up again.

    438 / 12.61 = 34.73 mpg
    Alright, well I have to admit this logic is flawed. The car has an 18 gallon tank, so the fact that I put 12.61 gallons in it before leaving doesn't mean I drove the 438 miles on that 12.61 gallons.

    Like I said, I'll be more particular when figuring the MPG on the beach trip in September. I'll make sure to use the DIC's "Average Economy" readout in addition to the other methods. However, I don't have to wait until then to compare the difference between the 'Average Economy' readout in the DIC vs. taking 'Fuel Used' dvided by 'Trip A' miles. I'm going to fill up tomorrow. Before turning the engine on after filling up, I'm going to:

    1) Reset the 'Fuel Used' to 0.0
    2) Reset the 'Average Economy' to 0.0
    3) Reset the 'Trip A' mile counter to 0.0

    It will probably be another 10 days or so before I need to fill up again, at which time I will do the following:

    1) Divide 'Fuel Used' by 'Trip A' mileage
    2) Compare the result from #1 to the 'Average Economy' in the DIC

    I'll report back here with my findings.

  6. #21
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    Re: Fantastic gas mileage on highway trip - 2008 DTS

    Unless needed for other reasons (only occurred once so far), I always use "Trip A" to track total miles on each tank of gas before resetting it. When filling the tank again, I calculate MPG directly from the gas pump total gallons to fill the tank divided into the miles traveled based on total miles in Trip A. This is a fool-proof way to calculate the most accurate MPG. After figuring, I reset Trip A for the next calculation of MPG at the next fill-up.
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  7. #22
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    There's no way to tell precisely how much fuel is left when you start to fill up, and there's no way to tell precisely how much fuel is in the tank when the pump stops. The AVG MPG readout is still more accurate.
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    Re: Fantastic gas mileage on highway trip - 2008 DTS

    Quote Originally Posted by MoistCabbage View Post
    There's no way to tell precisely how much fuel is left when you start to fill up, and there's no way to tell precisely how much fuel is in the tank when the pump stops. The AVG MPG readout is still more accurate.
    Don't know if you intended this for me or another poster. I do know that the mileage traveled via the "Trip A" is directly from the odometer, so is precisely how many miles traveled since last fill-up. Refilling the tank tells me how many gallons purchased. Divide number of gallons purchased into miles traveled from Trip A and you arrive at the most accurate MPG. My "Fuel Used" displayed on the DIC is never in sync with the gallons to refill. How could you possibly dispute the hard math that is self-evident to calculate MPG based upon gallons used? Has nothing to do with in-car DIC calculations. Has everything to do with number of gallons purchased divided into miles traveled based upon odometer reading. Simple math. I understand your point, but will continue to believe my own calculation is superior to that on the DIC. Over time, this will give the more accurate account.

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    The trip meter is accurate, but gallons purchased doesn't tell you exactly how many gallons were used since you reset the trip meter, unless you're letting the tank run bone dry. As posted, there's always going to be a different amount of fuel left when you start filling up, and the pump doesn't always fill the tank to the same level.

    It's splitting hairs, the traditional method is pretty accurate, I'm just saying the DIC readout is more accurate.

    Go back in this thread to see how the DIC arrives at its number, and why it's more accurate.

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    Re: Fantastic gas mileage on highway trip - 2008 DTS

    Quote Originally Posted by MoistCabbage View Post
    The trip meter is accurate, but gallons purchased doesn't tell you exactly how many gallons were used since you reset the trip meter, unless you're letting the tank run bone dry.

    It's splitting hairs, the traditional method is pretty accurate, I'm just saying the DIC readout is more accurate.

    Go back in this thread to see how the DIC arrives at its number, and why it's more accurate.
    No thank you, my method suits my purpose over the long haul. Splitting hairs over MPG is not necessary for either of us, unless one of us is determined to have the last word or has a deep-seated need to prove a point. I defer to you, if you wish the last word. No offense taken or given. Have a great week ahead.

  11. #26
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    ??? Where'd that come from?

    It has nothing to do with a need to prove a point or get the last word. I even said it was splitting hairs, and the traditional method was accurate.

    Many people don't know how the readout on the dash is calculated (any car, not just Cadillacs), and/or think that the reading is altered to make the vehicle appear more fuel efficient, so they waste time and effort doing the math, only to arrive at a number that is less accurate than the number they could simply look at on the dash. I explained earlier in this thread, and elsewhere on these forums, how the number is reached, so that they can trust the number, and save time (if they choose to).

    Quote Originally Posted by ronm01
    I understand your point, but will continue to believe my own calculation is superior to that on the DIC. Over time, this will give the more accurate account.
    Your post is a perfect example of why I explain it in the first place. You don't trust the number. I'm not trying to force anything on anyone, just offering facts for anyone looking.

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    Re: Fantastic gas mileage on highway trip - 2008 DTS

    Quote Originally Posted by MoistCabbage View Post


    ??? Where'd that come from?

    It has nothing to do with a need to prove a point or get the last word. I even said it was splitting hairs, and the traditional method was accurate.

    Many people don't know how the readout on the dash is calculated (any car, not just Cadillacs), and/or think that the reading is altered to make the vehicle appear more fuel efficient, so they waste time and effort doing the math, only to arrive at a number that is less accurate than the number they could simply look at on the dash. I explained earlier in this thread, and elsewhere on these forums, how the number is reached, so that they can trust the number, and save time (if they choose to).

    Your post is a perfect example of why I explain it in the first place. You don't trust the number. I'm not trying to force anything on anyone, just offering facts for anyone looking.
    Since you have asked, I will reply without trying to make it appear as though I want the last word. In response to your question, please accept my apology for my awkward and unnecessary end to my last post: "unless one of us is determined to have the last word or has a deep-seated need to prove a point." It s clearly evident to me that you wish to impart information concerning the accuracy of the DIC display. Thank you for drawing my unnecessary assumption to my attention.

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    Quote Originally Posted by ronm01

    Since you have asked, I will reply without trying to make it appear as though I want the last word. In response to your question, please accept my apology for my awkward and unnecessary end to my last post: "unless one of us is determined to have the last word or has a deep-seated need to prove a point." It s clearly evident to me that you wish to impart information concerning the accuracy of the DIC display. Thank you for drawing my unnecessary assumption to my attention.
    No hard feelings
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    Re: Fantastic gas mileage on highway trip - 2008 DTS

    Alrighty folks, I decided to go ahead and fill up again today.

    After I parked at the pump, my DIC readouts were as follows:

    Trip A: 249.7 mi
    Fuel Used: 13.4 gal
    Average Economy: 18.5 mpg

    As I mentioned previously, what I did before was take "Trip A" and divide it by "Fuel Used" in order to get mpg. Let's do that here:

    249.7 mi / 13.4 gal = ~18.6 mpg

    So the difference between the "Average Economy" and calculating mpg manually using the "Trip A" and "Fuel Used" is insignificant.

    The meter on the pump said I bought 14 gallons. That's a notable discrepancy, but I do agree with MoistCabbage that the pump doesn't necessarily always fill the tank to the exact same level.

    Like I said, I will post back here in September after the beach trip to let you know what kind of MPG I get.

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    Re: Fantastic gas mileage on highway trip - 2008 DTS

    Our fuel tanks have an air volume of about 1.5 gallons at the top of the tank to allow for fuel expansion and contraction with temperature - and that volume change is significant.

    That air volume is one of the factors that clicks off Pump A at so much fuel backflow and Pump B at another. The only way to totally fill a fuel tank (nowadays) to the max is to slowly trickle in that last 3/4 to 1 gallon of gas.

    The DIC indication of fuel used is very, very accurate, especially for quickie one-time fuel use looks. If you were to keep paper records over 5,000 miles or so you would then have a pretty good calculation of your average fuel consumption.

    I can fill the STS and spend a week in and around DC and wind up with an average 13 mpg - one "tank" of gas. I can then fill up and head down I-95 for SC and get 24.8 mpg on that tank............... but the average for the two tanks is 18.9 mpg. It takes a LOT of long term highway driving to pull that long term average DIC figure up into the 20's. I never reset my average fuel mileage figure - I do reset trip miles and fuel used readouts. Right now, mostly rural driving, the DIC says 19.3 mpg long term average.
    tjwhite721 and jazoo like this.

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