How accurate is your in-car mpg display?

Discussion in 'General' started by vtec-e, Sep 16, 2008.

  1. vtec-e

    vtec-e Celtic MPG Warrior

    After i told some workmates about my 1.5L/100k run in the diesel cee'd over 10 minutes they more or less laughed at me and said i'd gone too far with my claims!
    How did i do this then?
    I drove it to work one night like everybody else. Hard and fast. Mpg wasn't bad in fairness. Then i came to the last few km to work and traffic slowed down. Roadworks had things down to 60kph and traffic was flowing well. My engine and gearbox were hot and i reset the mpg display. I then hypermiled the a$$ off the car (dwl (i think!), nice-on coasting) at an average of 60kph and turning in the gate to work my average was 1.5L/100k. I was astounded to say the least!
    I've been told it's not possible and if people in mpg marathons can only get 76 imp mpg then i should shut up talking rubbish! etc etc....
    My question is this: How accurate should i expect the mpg display to be?
    Does it get more inaccurate as you go slower, as in my case above?

  2. lightfoot

    lightfoot Reformed speeder

    My Insight's tank figures have always been very close to the numbers on the display (I use Trip A for my tank tripmeter). Max difference has been about 3mpg out of 80, but often much closer.

    Obviously both measurements rely on the same odometer data. So what is compared is the car's measurement of fuel usage vs the pump. I figured some of the variation might be pump-to-pump and filling to different heights (even though I always stop at the first click).

    Can's say what is for other cars, or for Insights driven differently from my usage.
  3. vtec-e

    vtec-e Celtic MPG Warrior

    Hi Lightfoot,

    I always figured the display should never be too far from actual measurements so your 3mpg discrepancy seems realistic.
    I measured a tank recently on the kia and the computer seemed to over read by about 4 mpg but i'd have to run a few more to get better accuracy.
    It seems the inaccuracy is down to differing opinions of a full tank!

  4. Yahma

    Yahma Member

    Depends on how your gauge is calculating the MPG. In my BMW 325i, the ECU actually powers an analog instaneous MPG gauge that uses fuel injector readings to give a very accurate instantaneous reading. The Digital trip gauge will then use these readings along with odo sensor to give accurate weighted average MPG. Was within 1% verified with pen/paper method and scanguage.

    I've heard some other cars use a simple vacuum tube to the intake manifold for instantaneous readings, or use simpler methods of calculating the average and can have an error of +/- 10%. As with anything... YMMV...
  5. laurieaw

    laurieaw Sorceress of the North

    i find mine to often read 4-5MPG higher than reality when i fill it....i am always disappointed when that happens, even after 3 years with the car.
  6. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi Ollie:

    ___1.5L/100 km is over 150 mpgUS and that would be a stretch for a diesel at steady state but a running start with some FAS’s to a stop, possibly? You would have to run a lot closer to a hundred miles to find out. The super diesel I drove would allow 2.3 to 2.7 L/100 Km at 45 – 55 mph reasonably steady state but I never saw anything near 1.5 L/100 km even in a full comp level P&G while it was with me.

    ___Good Luck

  7. vtec-e

    vtec-e Celtic MPG Warrior

    Hi Wayne,
    Yes, the running start to the test was key to the high reading along with the hot engine, lucky with green lights etc. I did the same the following night and "only" got 2.5L/100k. Once you get down to tiny fuel consumption, anything will change it and a lot at that. I drove the kia to work last night and stuck to normal rpm(2-2.5k), and for the test route, my speed was 60 kph in 5th but consumption was much higher, around 3.5L/100k. Seems to me that the lack of heat in the engine played a part but i can't be sure. I'd like to try the hotter thermostat but it all depends on the service guys and the warranty (7 years...i thought it would be a good thing but now i'm thinking otherwise) I've asked around and got nothing useful. I'll be heading down the country this weekend with the family so i'll get to try out some cardboard in the radiator to reduce the cooling. I'm not too concerned as i've never heard or seen the cooling fan come on. I'll post the results here when i get back.

  8. vtec-e

    vtec-e Celtic MPG Warrior

    Well, the trip went better than expected. Seems the cee'd likes the radiator blocked right up. I've blocked it by about 60-70% and the engine gets up to temp quickly.
    Cosumption over the trip averaged at 4.5L/100k which is ok i suppose considering i had the family with me and the trunk full up to the top. At 80kph it was doing 3L/100k and seemed to do the same at 100kph too. Go figure. I did notice the fuel use increase while in traffic. I'm thinking the intercooler wasn't cooling as well as it should be because when i got out in the open with nobody in front, things improved. I noticed the fuel use dropped significantly when down to 60kph and hovered around the 1.5 to 3L/100 mark as the road went up and down. It was mostly flat though and there was only one big climb to 1000 feet above sea level which took about 10 minutes, if that gives you an idea of the kind of hill i'm talking about. That destroyed the mpg for sure!
    Anyway, the fuel use at 60kph stayed at the above levels for the duration of my times at 60kph so i'm confident that it will not need to be driven hard to get the heat into it. I heard the fan come on once while coming up to a red light in traffic and it just went green so i idled the engine instead of turning it off. The temp needle never went above where it normally goes so i'm a little puzzled about that as the engine was hotter. The only downside was that the AC was almost not working as the AC radiator is in front of the engines radiator and both are the same size. I only had the AC on for a very small time today as it was 19 to 20c (68f) Thats a cold day for some of you guys but we were sweating in it!
    In any case, it seems the hotter engine theory applies to diesels much more so than gasoline engines. Mpg gains seem to be big with a hot diesel and i'll be asking my service guys about the hotter thermostat.


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