Fuel economy meter accuracy
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Cadillac DeVille 1985 to 2005 including:
1985-1992 Fleetwood, 1993 Sixty Special, All FWD Forum Discussion, Fuel economy meter accuracy in Past Cadillac Vehicle Discussion; How accurate is the display that shows the "AVG" fuel economy in the left hand portion of the digital display? ...
  1. #1
    Deltamaster is offline Cadillac Owners Member
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    Fuel economy meter accuracy

    How accurate is the display that shows the "AVG" fuel economy in the left hand portion of the digital display?

    How accurate is the immediate DIC display of fuel economy?

    Driving around today I was getting an average fuel economy of 23 MPG at 55 miles per hour on flat and level roads.

    The DIC displayed in a rolling scale between 23 and 35 MPG during the same trip.

    Depending on the throttle position and power demands the DIC displays between 7 and 60 MPG.

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  3. #2
    rodnok01's Avatar
    rodnok01 is offline Cadillac Owners Connoisseur
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    Seems to be fairly accurate, the longer you go without resetting the avg the more accurate of long-term mileage it will be. The instant is what you see jumping around, put your foot into it and it will go to 3...

  4. #3
    MoistCabbage's Avatar
    MoistCabbage is offline Cadillac Owners 10000+ Posts
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    They're both dead on accurate. The PCM gets to those numbers by monitoring distance traveled, and the amount of fuel being delivered through the injectors.

    The AVG readout is more accurate than manually calculating MPG by the odometer and fuel pump readouts (as you never have the same exact amount of fuel in the tank when you stop to fill up).

    The instant readout is accurate, but it's information is pretty useless.

  5. #4
    Deltamaster is offline Cadillac Owners Member
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    Re: Fuel economy meter accuracy

    That means I get about 22 MPG traveling 55 MPH on straight and level road. The same road driving my Dakota, the meter reads a little over 17 MPG.

  6. #5
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: Fuel economy meter accuracy

    I average 26+ on the highway at 70. At 55 you should do even better. I'm talking long road trip here.

  7. #6
    The Donald is offline Cadillac Owners Fanatic
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    Re: Fuel economy meter accuracy

    Quote Originally Posted by Ranger View Post
    I average 26+ on the highway at 70. At 55 you should do even better. I'm talking long road trip here.
    I get 26 @ 80 mph on a loooong trip 500 mi or more..Got that last summer and covered 4200 mi..It seems to me in order to get a real accurate reading you have to travel 400 or more miles.. It takes a long time to get accurate gas mileage.. 100 mi wont do it...

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    Submariner409's Avatar
    Submariner409 is offline If it won't run, stick on fender vents
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    Re: Fuel economy meter accuracy

    The long-term average gas mileage saves a LOT of data points, and the longer you crawl around town the average mileage drops and it takes a LONG time to get it back to maximum during a highway trip. Your base Deville is probably getting 26 - 29 mpg on 87, long-term highway.

    Look at it this way: You fill up and reset the mileage and fuel readouts. You start up and immediately hit the Interstate for an hour at 65 mph - your long term average ratchets up to 28 mpg pretty quickly (few data points). You get off the Interstate and are quickly stuck in a traffic jam for one hour - your gas mileage is then 0 mpg and your average for the 2 hour drive is - - - 14 mpg. You stop & shop, then hit the Interstate for the trip home. It will now take longer to get that average back to 28 mpg - more data points in the calculation.

    The instant fuel mileage thing is yet another way to look at manifold vacuum effects - low vacuum = low gas mileage / high vacuum = high gas mileage.

  9. #8
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: Fuel economy meter accuracy

    It all depends on how you start out. If you start without zeroing it out it will take longer to average out. Zero it out and you'll have a good average for THAT trip.

  10. #9
    Deltamaster is offline Cadillac Owners Member
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    Re: Fuel economy meter accuracy

    That is what I did... zeroed it out before I left. That is a nice feature that is not available on most vehicles meters. My pickup does not have the option to zero it out.

    Really irks me that the big caddy V-8 gets so much better gas mileage than the Dodge Dakota V-8.

  11. #10
    Submariner409's Avatar
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    Re: Fuel economy meter accuracy

    Actually, it's not a "big Caddy V-8". It's a little 279 c.i. engine, slightly smaller than an early 60's Chevy 283...... and the tuning of the entire Northstar has it getting decent fuel mileage at about 1800 - 2400 rpm, but no real power until over 3,500 rpm - it has to wind up to make woof.

    A big Caddy V-8 was a 472 or 500.

  12. #11
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: Fuel economy meter accuracy


  13. #12
    Tailfin's Avatar
    Tailfin is offline Cadillac Owners Enthusiast
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    Re: Fuel economy meter accuracy

    The origianl question was the accuracy of these things. All things being equal, they are very accurate...but a few things can make them inaccurate. I had a 90 Deville which had an inaccuarate one, and after lots of postings on here, and throwing parts, the problem was the injectors. The fuel pressure regulator is another thing that can throw it off, but that's rare, and easily checked at the vacuum hose. If you find you're filling up and the pump is telling you VERY different from the fuel data center, the problem is injectors...an expensive fix usually :-P

  14. #13
    Deltamaster is offline Cadillac Owners Member
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    Re: Fuel economy meter accuracy

    Thank you all for your insight. It SEEMS to be fine, though I am not positive. Good enough for government work, though.

    Have a Merry Christmas everyone!

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