fuel saving, coasting - vs - in gear

[culzean]

Has anyone got any personal experience of the amount of fuel savings in the continuing debate about whether 'coasting' out of gear is better for economy than 'leaving the car car in gear' when going down a hill?

Exponents of the 'leave it in gear' debate say that modern engines will cut off the fuel entirely when the car is left in gear and that will result in maximum savings - surely this ignores the losses caused by engine pumping, gearbox and drivetrain friction, which can be considerable (this is why you leave the car in gear when parked, to assist the handbrake).  You will go down the hill slower and travel less distance on the flat or 'up the other side' until the point where your speed drops to a level where you have to put the power back on.

People who 'coast' will say that the only losses you have when coasting are wheel bearing, wind drag and tyre drag (which you still have if you leave it in gear anyway). And your speed will be greater (subject to speed limit of course) so the car gets more 'free gravity' energy and travels further on the flat or up hill before the speed drops enough for you to have to engage gear and apply power.

When you 'leave it in gear' the gearbox and drivetrain are absorbing the power trying to slow the car down, and are subject to wear - and brake pads are cheaper than gearboxes!

All I know is that with a freshly zeroed trip on my Jazz GD 1.4SE that the 'mpg' reading goes up to 150mpg whether I coast or leave it in gear, but I suspect that may be the highest figure the computer can handle.

By the way many people think that 'coasting' is illegal in the UK, but it doesn't appear to be.

[Geoffers]

Sorry, but I have better things to do than worry about wether that saves petrol or not!
Some people have too much time on their hands!

[guest765]

If it has got that bad you have to coast to save petrol in a Jazz ........then get rid of the car you cant afford to run one......... 
Wanna buy my 4ltr Daimler Super v8.........automatic ? now theres a car to try and coast in.........

[JazzyB]

'Coasting' out of gear in illegal in the UK and I would say pretty much anywhere else in the world.

You have to be in total control of your car therefore the car must be in gear whilst driving.

Going down say a steep hill out of gear and using the brakes to slow down would probably cause them to overheat.

So keep it in gear 

[culzean]

'Coasting' out of gear in illegal in the UK and I would say pretty much anywhere else in the world.

You have to be in total control of your car therefore the car must be in gear whilst driving.

Going down say a steep hill out of gear and using the brakes to slow down would probably cause them to overheat.

So keep it in gear 

has anyone looked at section 122 of the new highway code lately? It appears that they mention coasting but do not in any way imply that it is illegal.

122
Coasting. This term describes a vehicle travelling in neutral or with the clutch pressed down. It can reduce driver control because
- engine braking is eliminated
- vehicle speed downhill will increase quickly
- increased use of the footbrake can reduce its effectiveness
- steering response will be affected, particularly on bends and corners
- it may be more difficult to select the appropriate gear when needed

Older versions of the Highway Code used to specifically advise against coasting. The current version, for some reason, doesn't.

[JazzyB]

'Coasting' out of gear in illegal in the UK and I would say pretty much anywhere else in the world.

You have to be in total control of your car therefore the car must be in gear whilst driving.

Going down say a steep hill out of gear and using the brakes to slow down would probably cause them to overheat.

So keep it in gear 

has anyone looked at section 122 of the new highway code lately? It appears that they mention coasting but do not in any way imply that it is illegal.

122
Coasting. This term describes a vehicle travelling in neutral or with the clutch pressed down. It can reduce driver control because
- engine braking is eliminated
- vehicle speed downhill will increase quickly
- increased use of the footbrake can reduce its effectiveness
- steering response will be affected, particularly on bends and corners
- it may be more difficult to select the appropriate gear when needed

Older versions of the Highway Code used to specifically advise against coasting. The current version, for some reason, doesn't.

Whether its illegal or not 'coasting' goes against everything I was taught when learning to drive.

Clearly its dangerous for the very reasons listed in the highway code.

[Geoffers]

I doubt that there is one Driving School that will endorse 'coasting'!

It is not a very clever way to save fuel, if it saves fuel at all.

[culzean]

I doubt that there is one Driving School that will endorse 'coasting'!

It is not a very clever way to save fuel, if it saves fuel at all.

Driving schools are there to get you through the test, only after that do you start to really learn how to drive, and hopefully get some judgement of your own based on experience as to when it is safe to do certain things. You obviously wouldn't attempt to coast down say Porlock Hill (1 in 4) but there are many hills that you can coast down very safely indeed and never exceed the speed limit or even touch the brakes.......

[Geoffers]

Personally I wouldn't 'coast' anywhere. It's stupid and it's asking for trouble.

[RichardA]

Coasting is not illegal as rule 122 is not accompanied by a MUST NOT. A police patrol could (with discretion) consider it driving without due care(?), etc., just like putting on your sunglasses or inserting a CD when driving.

Personally, I wouldn't do it and I doubt it would really make any difference to fuel savings.

Edit: on a similar note, please don't be tempted to creep down a slight hill whilst in a stop/start traffic queue without the engine on. Someone in a Triumph Spitfire years ago went into the back of my Dad's Proton doing just this as it caused her brake servo to dry out and make the brake pedal harder to press. Of course, I gather the Spitfire had no steering lock so this would be almost impossible in a modern car.

[culzean]

Thanks Richard A for bringing a calming influence to this subject and being the only one not to imply that I am too stupid to be let loose on the road for merely asking a question about improving fuel economy - I have coasted down suitable hills and it does make quite a difference to fuel consumption.  I am sure one of the main reasons why most people buy a Jazz is because of its fuel economy and yet when someone suggests a way of saving even more fuel and thereby lowering CO2 output they are derided.

Apart from the fact that it would be impossible for the police to tell if you were or had been coasting or not (unless they could look into your moving car and see that you were in neutral) at least we have established that it is not illegal per se (as 90% of drivers seem to say when they are asked, but have never actually researched the matter), so hopefully it has been informative.

By the way, this subject has and is being discussed on many other car forums around the world, with many posters saying that it does make quite a difference to fuel consumption, but should be used with judgement - ie. don't coast down a twisting mountain road.

I would now be quite happy for you to delete this subject from the Jazz forum and I promise never again to introduce anything soooh revolutionary as an easy way of saving fuel - but maybe when fuel hits the £3 per litre price some of the nay-sayers may even find themselves considering it!!!!!

[nowster]

Coasting downhill in neutral won't help with a Jazz. In neutral, engine management unit will have to feed enough fuel into the engine to keep it idling (AKA tickover). If you're in top gear with your foot off the accelerator (or on cruise control), the EMU can cut the fuel feed completely.

[guest238]

A common misconception is to think the highway code is legally binding. Only certain parts are defined as law and are identified as either 'MUST' or 'MUST NOT' and contain a reference below in red to the Road Traffic Act (legal document). The rest is merely 'advisory', but maybe used against you by the police as basis for careless driving, driving without due care and attention, etc, etc, etc, or by the insurance companies when appointing blame after a collision.

As for coasting? I can't see much benefit - much like BMW/MINI's efficient dynamics!

[guest869]

I was wondering exactly this, does coasting use less fuel than leaving it in gear?

The advantage of coasting is less rolling resistance so you can go further before having to reapply the throttle.

Does the ECU cut off the fuel supply completely when off the throttle and in gear?

It may depend on the hill and what is beyond the hill. If the hill is steep with a give way at the bottom, then leaving in gear (assuming NO fuel is being delivered to the engine) will result in better MPG. If the hill is gentle with a clear roll out then coasting may give better MPG.

It is probably a question of judgement.

I can not see a problem in coasting in certain conditions. Surely it doesnt matter what we were taught as we surely can make judgement calls ourselves....... though the way motoring law goes I suppose we are all deemed not able to use judgement anymore when driving and have to have everything layed out for us...........