How much fuel does an engine burn driving downhill? | Auto Expert John Cadogan

!!! NEW THUMBNAIL !!! (Same old smartarse...)
Pro tip: The answer to the 'how much fuel...' question is 'zero' (usually, if the hill is steep enough and if your speed is over some minimum threshhold).
Here's the back-story:
QUESTION: "When my car rolls downhill in gear without accelerator input, the instantaneous fuel usage shows 0.0L/100km. However, if I flick to neutral and simply roll, it will read from 0.6 to 1.1L/100km. The engine is definitely turning and firing so it can't possibly be using the reported 0.0 but the fact all my recent vehicles have said this makes me wonder what's going on. Is there something about being in gear that miraculously uses less fuel than when in neutral?" - Jason
ANSWER: When you select neutral and roll downhill the engine is idling. Idling consumes fuel, obviously. The engine still has a job to do. It has to overcome resistance, compress the charge and drive the ancillaries. Fuel is required to do that - though not very much.
When you are in gear on a closed throttle and headed downhill the engine is being turned by gravity (via the wheels and transmission/driveline). In this state (depending on the car) fuel is shut off, hence the zero reading.
Your statement: “…the engine is definitely turning and firing…” is almost correct, but it’s not receiving any fuel, so it’s only ingesting air. Only the spark plugs fire. There’s no fuel/air charge to ignite. When you open the throttle, fuel is introduced, the engine fires normally and produces mechanical work that moves the car.
Jason followed up...
QUESTION: "I did not know an engine could fire without fuel. Always presumed there had to be a fuel to air mixture of some sort for there to be any kind of bang. A good day is any day you learn something and today I have, so thankyou." - Jason
ANSWER: The plugs fire. There’s no charge in the engine to burn in this ‘closed throttle’ state, coasting downhill. Just so we’re on the same page here, the engine is not firing in the conventional sense. It’s being turned over, with zero fuel, thanks to gravity.
QUESTION: "Is it bad for the plugs to fire when there's no fuel, particularly for an extended period? I recently used the engine to brake the car in this way all the way to the bottom of a mountain. My trusty trip computer said this was costing me nothing so I was happy to roll with it. I'm now wondering if this extended amount of engine firing with zero fuel was in any way bad for the plugs or the cylinders?" - Jason
ANSWER: You’re over-thinking this. Dozens of engineers worked on this powertrain in R&D. The spend millions of the company’s money reliability testing. If it was bad to do this, they wouldn’t do it. Thought experiments are fine, mate, but they have to be underpinned by reason. It’s nuts to speculate that this might be bad, in the absence of any evidence or reasonable grounds to suspect a problem (such as a foreseeable failure mechanism flowing from operating in this way).
If the spark plug fires in air, or in air-fuel, all that happens is electricity jumps from one electrode to the other. If there’s fuel in the mixture, it burns. If not, the engine is just an expensive air pump, driven by gravity. The engine’s still being lubricated in that state, so it could operate like that indefinitely.