A counter point: In my previous car the screen showed 0.0 when idling, which obviously was false (as the engine was still running). It should probably show infinite instead. (km/l for 0 km).

Some cars show liters per hour when stationary, which always seemed logical to me.

Or change to l/h when you're stationary or with the clutch disengaged.

Of course it works that way, but just for the note: what the car computer reports in dash is manipulated by software and it can be anything. It's not necessarily completely true.

Simple understanding on how a car works - "wheels turn the engine" when in gear, otherwise fuel is consumed when in neutral..

Not so simple. Until recently, a standard ICE couldn't stop injecting fuel while coasting (unless you turned the car off, which brings other problems...).

I remember when I was learning to drive being taught about engine braking (with an automatic) and the tradeoff between using up the brakes or burning fuel.

For every fuel-injected engine the ECU should cut injection when coasting. So "recently" in your case applies to carburettors which will behave as if the engine were idle and thus still feed fuel to the engine. I'm not sure 25 years ago counts as "recently".

> For every fuel-injected engine

Well, I own a 911 that says otherwise. It's an older fuel injection system (Bosch K-Jetronic), but it's most certainly fuel injection. Porsche wasn't the only maker to use Bosch fuel injection.

I just don't think DFCO (deceleration fuel cutoff) was a feature of most of the early electronic fuel injection systems. Perhaps it was standard by the time EFI made it to economy cars. I'm fighting a massive urge to go down this historical rabbit hole and read about it, but there's work to be done today.

another anecdata: I once drove a car that reported negative fuel consumption when driving down the hill.

EDIT: no, it was neither electric nor hybrid car, but some old petrol car with manual transmission (Saab 900 or similar).

Electric cars generally do that ;).

A hybrid?