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?