Surely that rather defeats the reason for having an automatic in the first place!

Not really, its no different from putting a manual into neutral going down a hill. Not that I would bother too much about doing this on a manual or an auto.

Thing is John, your comparison between yours and your wife's average economy does not, by itself, support continuing to use this technique. The difference could easily be accounted by her simply being harder on the throttle pedal. What you need to do, now that you have your average with this hypermiling technique, is to spend a few months driving as you would (in terms of acceleration and revs management) but without putting it into neutral.

I do that in a manual,

IIRC the Highway Code disapproves, but I started doing it after passing my test, sort of as a reward, and have done it ever since,

I developed the technique in Israeli occupied Palestine in the 80's, due to a combination of hilly terrain, uncertain petrol supply, and a non-servo braked VW Beetle which made cutting the engine on downgrades less fraught

.My longest engine-off coast is the Rest and Be Thankfull pass in Scotland, a fast run all the way down to Arrochar if you can avoid braking where the road crosses the burn about half way down.

I assume the highway code disapproves because you dont have access to "instant acceleration" which could perhaps sometimes get you out of trouble. OTOH I can think of at least one incident where a lack of lift off braking probably saved me and a couple of other people from death or serious injury

www.lightfoot.co.uk/news/2018/02/02/bad-technique-.../

I've practised this since the '80s when I used to compete in non-observed economy runs. I couldn't figure out how others were getting so much better mileage and then someone put me right. Once got over 90MPG from a Maxi.

I don't now switch off the engine after an incident where the steering lock operated! I also believe it's not good practice to bump start a car fitted with a cat.

Having owned several Maxis I still find these exaggerated figures hard to credit. But never mind the steering lock, the engine should never be stopped if your car has a brake servo !

the engine should never be stopped if your car has a brake servo !

Why not? There's at least a couple of assisted operations from the vacuum reservoir and as long as you bear that in mind you should be OK. I have been - so far.

Coasting in neutral means you don't have engine drag to slow you down. If the revs are above 1200-1500 with the throttle off, no fuel will be used (unless you've one of these snarly, barky, noisy things that continue to fuel). I use both methods, depending on the circumstances.

'Coasting in neutral' means out of gear with the engine running - at least to me. Free-wheeling with a dead engine is perfectly possible (I have done that too) but not only do you soon empty the vacuum reservoir, you lose PAS as well, unless it's electric.

Current car - Peugeot 308 auto - shows 999mpg on the mpg readout when "coasting" downhill. I assume the fuel supply to the engine is simply stopped on the overrun.

On a modern manual vehicle there may be a balance point where the fuel consumed by idling and coasting in neutral is less than that used to regain speed lost through engine braking if in gear - but this would be dependant on the gradient and safe speed.

Go back four decades and carburettors were the norm. They functioned through a vacuum created in the inlet manifold which "sucked" petrol from the carb into the engine.

Coasting in neutral may then have been more economical. Leaving it in gear would create a greater vacuum (more "suck") as the engine would be turning at greater than idle speed.

That seems to be what the claim is. With modern ECU and fuel injection I am not convinced.

Let me see if I have things straight. Descending a hill with the car in neutral with fuel being burnt to keep the engine ticking over is more economical than descending in gear with the supply of fuel cut off? Right?

It depends. In neutral you'll probably reach a higher speed and have more momentum to carry you further.

In gear (if the revs are above a certain range, in my experience 1200-1500) the fuel will be cut off, but you'll have the drag of the engine, which on a slight gradient may actually slow you.

On a steep hill you probably want to leave it in gear for the benefit of engine braking. I can think of one hill on the A30 where you can be exceeding the the NSL in gear.

Let me see if I have things straight. Descending a hill with the car in neutral with fuel being burnt to keep the engine ticking over is more economical than descending in gear with the supply of fuel cut off? Right?

Right. In the first instance, at the bottom of the hill the car will be travelling much faster than in the second instance, and so will possess much more kinetic energy, which will more than compensate for the tiny amount of fuel energy expended in keeping the engine ticking over at very low revs. (Also, once the revs drop below a certain level, the fuel overrun cut-off is disabled).

And the electronic pre-programmed artificial readings of 120mpg and 999mpg mentioned in above posts are irrelevant red herrings.

It seems many, possibly those who have never used a starting handle, still cannot grasp the simple fact that the faster you make an engine spin, whether by injecting fuel or by using the kinetic energy (motion) of the car, the more energy is needed. The kinetic energy employed on the downhill run has originally come from the fuel required to attain the top of the hill in the first place.

Try a simple experiment with a bicycle pump; stick your finger over the end to prevent air escaping, and pump against the resistance about fifty times, compressing the air as much as possible. Each compression generates heat owing to the energy expended by your now tired arm, and your hand with the occluding finger will feel that heat as the end of the pump gets hot. This is what happens to a much greater degree in the compression stroke of a four stroke engine's pistons, and is why they are so effective as a brake when un- or under-fuelled. The engine braking energy is dispelled as heat via the exhaust pipe and cooling system.

I realise that all this might be difficult to understand by those who failed to achieve a reasonable grade in GCSE Physics, but I have tried to describe it as simply as possible.

Edited by John F on 10/07/2021 at 10:39