Neutral downhill using less fuel a myth

[MY95]

So ive been using neutral going downhill as the speed stays the same or climbs more compared to always decreasing in gear downhill in the 306, what is the science behind it? Most people say it doesn't use any less fuel. Does it use more, less or the same fuel? I know engine braking uses less fuel than using the brakes.

[Piggy]

Closed throttle, in gear, uses almost no fuel. Idling in neutral uses fuel

[bashbarnard]

depends upon the way the car is fuelled. take for example the xud vs the dw10. the dw10 will see when youre rolling with the throttle on slightly and inject less. the xud cant tell the difference and will inject said amount wherever your throttle is sitting. so dw10 no, xud, yes.

thats my 2p on this. you have to remeber that every car is diffrent

[allye]

Modern engines supply zero fuel after a certain rpm with zero throttle. Remember reading 205 GTI's was around 2000rpm (not modern I know)

But yes, rolling With no gear selected uses more than rolling in gear.

[Danny Wideboy]

Dude, go to the pub.


Stop thinking about MPG. it's pointless,.

[Paul Baldwin]

Depending on the car and how new it is this may well be the case.

However, neutral, downhill , means no engine breaking and limited transmission losses, depending on the terrain you are travelling over you may well be able to use the inertia of the vehicle to overcome some gradients that would be totally impossible with the engine at closed throttle in gear. Therefore it is possible to save fuel by coasting.

The grey area of legalities on doing it however are another matter....

[Ruan]

Depending on the car and how new it is this may well be the case.

However, neutral, downhill , means no engine breaking and limited transmission losses, depending on the terrain you are travelling over you may well be able to use the inertia of the vehicle to overcome some gradients that would be totally impossible with the engine at closed throttle in gear. Therefore it is possible to save fuel by coasting.

Yeah this...

XUDs and anything rotary pumped do have fuel cut off (pretty much - the IQ goes down to a small fraction of that on idle, literally barely measurable) too... HDis too and pretty much any fuel injected car in the last 30 years.

The pumping losses sometimes cannot overcome the minute quantity of fuel being injected... However lightly placing your foot on the throttle to maintain speed is likely to be just as efficient as the engine running at idle anyway...

Basically yeah, stop worrying about it.

[Dum-Dum]

In ye olden days when everyone's cars had carburettors' it was probably true but not these days.

[Paul Baldwin]

In ye olden days when everyone's cars had carburettors' it was probably true but not these days.

Next time you're in a newish car traveling down an incline at a steady rate with your foot on the throttle to maintain momentum dip your foot on the clutch and tell see how many rpm you are doing above idle :-)