Do I use less petrol at lower revs?

[Deleted member 26715]

[QUOTE 2775309, member: 9609"]only have it in gear when you need to accelerate or de-accelerate the car.[/quote]
This is now incorrect with the more modern car that supports an ECU, when the car is pushing the engine along, the modern ECU will switch off the fuel completely from the injectors.

Alan...

 

[Pale Rider]

With the clutch in the engine has to power itself at idle to keep turning over . If you leave it in gear a modern engine can turn the fuel down to a dribble and the movement of the car keeps the engine turning round.

Or something like that.

That is correct, the engine management system turns the fuel right down when going down hill in gear, having said that I reckon the difference in consumption compared to coasting in tickover will be minimal.

But as others have said, coasting is undesirable from a safety point of view.

 

[Deleted member 26715]

[QUOTE 2775340, member: 9609"]But spinning the engine on the over-run takes a vast amount of energy and the car will slow down rapidly, then you will need to use more fuel to get going again.
If it is a long gentle downhill slope coasting with the engine idling will be far more efficient, until the car unexpectedly looses control.[/quote]
I'd like to see the paperwork

Alan...

 

[donnydave]

[QUOTE 2775340, member: 9609"]But spinning the engine on the over-run takes a vast amount of energy and the car will slow down rapidly, then you will need to use more fuel to get going again.
If it is a long gentle downhill slope coasting with the engine idling will be far more efficient, until the car unexpectedly looses control.[/quote]

depends what gear your in. If you're in a low gear engine braking will be more pronounced. Depending on whats ahead this may be desirable, or if you just want maintain speed downhill, or maybe scrub off just a little speed, leave it in a higher gear

edit: low gear and high revs, as opposed to high gear and low revs

 

[XRHYSX]

[QUOTE 2775309, member: 9609"]That depends, costing downhill to a junction then needing to use the brakes will use more fuel - however coasting downhill when there is no need to brake will be more efficient.

Most efficient driving will come from predicting the road ahead so that you never need to use the brakes, and only have it in gear when you need to accelerate or de-accelerate the car.

And the highway code advises you should never coast, as your car may unexpectedly loose control at any moment - so clearly not worth the saving![/quote]
This

LOTS unless it is of a DSG type, autos are known for inefficient use of fuel due to the 'slushbox' nature of the torque convertor as there is no direct drive from engine to wheel.

Alan...

In my lorry a Volvo FM I have a proper 12 speed g/box with a clutch but no clutch pedal, is that the same as the DSG box you get in the VW/AUDI group

 

[donnydave]

[QUOTE 2775354, member: 45"]So why does the mpg computer shoot up when you knock it out of gear?[/quote]

because its using less fuel than if you were maintaining speed in gear? The compooter reading would no doubt go up in a similar fashion if you simply lifted off the throttle on a long descent, unless your in something racy that's dumping great wads of fuel in on the overrun to make sure you get going sharply when you next stamp on the throttle

 

[asterix]

My yaris has one of those arrows that tell you to change up or down. On hills where the engine is coping fine, it nevertheless tells me to change down on some occasions. Similarly if there is a strong headwind the same happens.

I have no idea how it works but it seems possible that under stress the engine demands more fuel than it burns or summat like that.

With my van the most important factor, above laden weight even, is wind direction. Being quite flexible in my planning, I always check this before a long journey (usually 750-800 miles) and choose a day with the right wind direction if I can.

 

[donnydave]

[QUOTE 2775375, member: 45"]That's not always the case. On a gentle downhill with my foot off the pedal the MPG still increases on the display if I take the car out of gear.[/quote]

Might depend how clever the engine is, or what air/fuel ratio limits it has. I'm afraid we've reached the limit of my knowledge!

 

[XRHYSX]

My lorry will kick itself out of gear when its on cruise at the speed ive set and it feels its not under load, but will jump back in if it goes over and use engine braking to slow or power to get back up to speed

 

[donnydave]

My lorry will kick itself out of gear when its on cruise at the speed ive set and it feels its not under load, but will jump back in if it goes over and use engine braking to slow or power to get back up to speed

Really?! Can this feature be adapted to cars to kick the driver's brains into gear?

 

[snorri]

[QUOTE 2775309, member: 9609"]
Most efficient driving will come from predicting the road ahead so that you never need to use the brakes, .[/quote]
I have always done this, can't break the habit, but every time my cars have required to have brake problems fixed they tell me the problem has arisen due to lack of use of the brakes, and I am urged to use the brakes more.
What I save on road fuel I spend on brake repairs, you can't win!

 

[gavroche]

On a level road, the optimum rpm to change to a higher gear is 2500rpm for petrol and 2000 rpm for diesel.

 

[tyred]

On a level road, the optimum rpm to change to a higher gear is 2500rpm for petrol and 2000 rpm for diesel.

I would say that it would depend on the engine to be honest, they're all different and tuned and set up in different ways, some are happy to rev to oblivion, some aren't.

 

[swee'pea99]

[QUOTE 2775491, member: 9609"]I think this will be the next leap forward in fuel efficiency, spinning an engine at 3000 revs for no real purpose is unnecessary waste. Cars may one day be like bikes, on downhill sections they will free-wheel with engine off.
But do wait until your car has been designed with this feature, or you may find yourself with braking and steering issues)[/quote]
Aren't there cars already that do this? I seem to remember reading about somesuch, and how unnerving the silence is, when you're driving along and the engine stops.

 

[tyred]

[QUOTE 2775491, member: 9609"]I think this will be the next leap forward in fuel efficiency, spinning an engine at 3000 revs for no real purpose is unnecessary waste. Cars may one day be like bikes, on downhill sections they will free-wheel with engine off.
But do wait until your car has been designed with this feature, or you may find yourself with braking and steering issues)[/quote]

Ecomatic MK3 Golf?