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My 2017 Grand Cherokee with the 5.7 "Hemi" V8 shows me instantaneous as well as average fuel consumption in a dashboard display. I am recording distance driven between refuelings and quantity pumped, from which I can also calculate fuel consumption.

When I calculate consumption (over 3 refuelings, ~1500 total km), I get about 14 l/100km, where the dash display shows about 13 l/100km. Ok, so the display would suggest fuel consumption is about 7% less than my calculation, perhaps not a big deal.

What I am really curious about pertains to the instantaneous fuel consumption. When the car is idling at a stop, it obviously pegs at 99 (has to show something and I guess the designers won't have it say "infinite"), but what surprises me is that when the car is braking/coasting down to a stop, (I typically see this in urban driving), the reading will sometimes go as low as 1 l/100km. Why this surprises me is that the car isn't moving all that fast and the engine is still turning. For years I was driving a 1998 with the 318 (5.2l) V8 and its instantaneous fuel consumption reading never went below 7.

What accounts for the low instantaneous value in my new car? Is fuel cut off and the engine kept turning via the transmission? Do newer engines require far less fuel to idle than they used to? Or are there tricks/quirks in the way the computer reports fuel consumption?

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  • It would make more sense (to me) to show 0 mpg at idle (and stopped rather than coasting) rather than 99 (or infinity). I mean, you're burning fuel, but not moving. :|
    – 3Dave
    Jul 10 '17 at 14:41
  • 99 is for litres/100km, so burning and not moving would be infinity.
    – Anthony X
    Jul 11 '17 at 1:15
  • Ah. We do the inverse - distance per volume.
    – 3Dave
    Jul 11 '17 at 4:41
  • @DavidLively Yeah, it can be confusing. Way back here in Canada we used to measure distance in miles and fuel in IMPERIAL gallons, so mileage was miles per gallon, but the numbers were higher than US because the Imperial gallon is a bit larger than the US gallon, but otherwise recognizable units to you folks. When we switched to metric and started measuring distances in km and fuel in litres (or liters), the weirdness happened - fuel consumption measured in litres per hundred km.
    – Anthony X
    Jul 11 '17 at 23:15
  • That actually makes sense. In games, people like to measure performance in frames per second, but during development, seconds per frame is much more valuable. The numbers don't jump all over the place when small changes occur.
    – 3Dave
    Jul 12 '17 at 19:28
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Most injected vehicles have "Over-run fuel cut" - Which cuts fuel almost completely when you are coasting or off the throttle down a hill.

You can often turn this off on after-market ECU's which gives you that POP/Bang/Crackle when you lift.

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Yes, the engine when used as an engine brake is given very little fuel - I think most of us would like those consumption figures all the time :)

The other not going below 7 may have been just how they calculated the averages...

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  • So the automatic will decide to go into an "engine braking" mode at times? It does occasionally feel like the car wants to slow down more than I expect from pedal use. It's not something I've encountered on older cars - they always used to "freewheel" when rolling along at idle.
    – Anthony X
    Jul 8 '17 at 19:39
  • Some automatics "lock" the fluid flywheel above a certain engine speed - for fuel consumption , cruising etc not sure exactly when this first became popular late 80's possibly
    – Solar Mike
    Jul 8 '17 at 21:39
  • I know about the "lockup" feature as a fuel saver during cruise, and I know even older automatics could engine brake but only if the shifter was selected to L1 or L2; in the normal D position, an overrun clutch would always allow the engine to drop to idle RPM. Seems newer transmissions act differently(?)
    – Anthony X
    Jul 9 '17 at 15:57
  • Technically, couldn't the engine be receiving no fuel when engine braking? The transmission will keep the cylinders pumping, no?
    – Spivonious
    Aug 9 '17 at 18:58
  • Very little , a weak mixture but just enough
    – Solar Mike
    Aug 9 '17 at 19:14
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Actually fuel consumption during idle mainly depends on the engine size, the more you have "volume" in the engine, the more it needs air/fuel mixture to keep combustion going. So 5.7L V8 will consume about twice as much as 2.8L inline 4 during idle, all other things being equal (actual consumption during driving depends on many criteria).

Almost all modern cars are "calculating" fuel consumption, although they have all the possibilities to provide the most accurate figures, I always saw them being on the positive side compare to real fuel consumption. Also on most cars I've seen fuel consumption during idling in Liters/Hour rather than Liters/Km. For instance 1.6L inline 4 consumes 0.6L/Hour during idle when fully warmed.

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