And if so does it affect the car in any other way, are there any side affects?

Not really but it does cause unnecessary tail backs, select the right gear and keep a comfortable distance.

by coasting, eg not having the car or vehicle in gear,there is no power to the wheels, thus swerving to avoid a pedestrain or animal who crosses in front of you will have no effect, just a loud reving of the engine.

Oh and its ilegal.

I cant really see how you can save fuel by coasting as your going to use as much fuel/energy getting back up to speed again,Turning the engine off at lights will save fuel. 😉

I have seen a few problems (Electronics) in rear wheel drive bmws where the ignition has been left on while towing on a beaver tail recovery truck 😥

On a modern car if you’re coasting out of gear or with the clutch dipped fuel is still being delivered to the engine to keep it turning over (idling). If you do the same but are in a gear, think going down a hill, the car is in effect turning the engine so the ecu doesn’t need to deliver any fuel. That’s my understanding anyway.

by coasting, eg not having the car or vehicle in gear,there is no power to the wheels, thus swerving to avoid a pedestrain or animal who crosses in front of you will have no effect, just a loud reving of the engine.

Not sure I unrest and your logic there ?

Are you saying that with the car out of gear you can’t steer (to swerve round an object)
Or that steering somehow causes the engine revs to increase ?

Coasting does save fuel but only if done sensibly & safely. I only coast if there’s no traffic behind me, & depending on the gradient/distance to junction etc. Main factors to consider are, traffic behind, speed, gradient & traffic in front. I know it works cos I can see my miles per gallon-ometer reading go up.

jeffl – Member

On a modern car if you’re coasting out of gear or with the clutch dipped fuel is still being delivered to the engine to keep it turning over (idling). If you do the same but are in a gear, think going down a hill, the car is in effect turning the engine so the ecu doesn’t need to deliver any fuel. That’s my understanding anyway.

I believe this to be correct, as long as the throttle is not used.

But if you’re in gear, the engine acts as a brake so you end up having to use the throttle to keep speed up surely, whereas in neutral you can actually pick up speed going downhill.

As has been said already, injection engines with slightly clever fuel maps for 0% throttle demand use no or very little fuel when off the throttle, less than if you knocked into neutral. Post 2000 for anything other than base model as an estimate. Older injection (basic fuel mapping) or carb engines will use less fuel at idle than at higher engine speeds coasting in gear.

I reckon coasting can be done in a safe way on the roads, but it is illegal/against the highway code. I occasionally do it to save fuel, but keeping an eye on traffic behind, in front, road conditions etc. Don’t see why it would contribute to an accident, my car is so gutless the accelerator pedal is useless in an emergency, steering or brakes are the only reasonable way out.

I had to do coast properly when my engine cut out after an alternator belt failure. Managed to get 2.5 miles on a dual carriageway with no engine that day to the next safe stop, was luckily pointing downhill. 😀

I engine brake generally not sure if it saves fuel but apropriate gearing down does help control your speed a tad better than banging the clutch pedal down and coasting with no gear…

the zero fuel use thing is correct but there is also a braking effect. going downhill the right gear choice and weight of the car will usually see no net speed loss. on the flat you will lose it slowly (also if you are coasting). And you cant really coast uphill either.

However, the real issue is the reduction in car control in an emergency. In an emergency you have 3 things you can do…steer, brake or accelerate. If you’re in neutral you only have two. On top of which a sharp or sudden turn could require balancing through the throttle so turning can be less effective.

That I suspect is why its against the highway code.

How much fuel are you looking to save? And could you not make these savings in other ways?

Best way to save fuel seems to be to drive between 55-60mph. My cars mpg-ometer thing has gone up from 36 to 44 mpg since doing this combined with accelerating slower.

However, the real issue is the reduction in car control in an emergency. In an emergency you have 3 things you can do…steer, brake or accelerate. If you’re in neutral you only have two.

Do you have to actually be in neutral to coast? Is that different to just having the clutch in?

If not then surely if you are in a sensible gear with the clutch down then you can still accelerate out of trouble relatively easily?

If I’m not watching what I’m doing I can get about 320 miles from a tank- a mix of motorway and urban.. If I keep the speed down to 55 on the motorway, and coast where safely possible I can get 400 miles from a tank..
Don’t understand the comments that coasting in gear uses less full than coasting in neutral though- mapping or not!

I look at it this way, you convert fuel into energy to accelerate the car if you use the brakes to slow the car down you convert the energy of the car into heat through thr brakes. If you want to save fuel, accelerate more gently and use the brakes less (by looking ahead and reading the road conditions). How many people do you see tear away lights only to slam on the brakes at the next set.

this thread is everything bicycling is not

jeffl – Member

On a modern car if you’re coasting out of gear or with the clutch dipped…

Why would you be hitting the clutch whilst coasting? In my mind, coasting is all about disengaging the engine completely with your feet off the pedals (usually downhill). And I do have power steering, but have discovered I can still steer (with my MASSIVE GUNS) when the system goes boobs skyward. And that’s in a huge Volvo.

Dibbs, yeah, that’s what I do.. Looking ahead, keeping my distance etc..
I can’t see how coasting downhill in neutral with revs about 800rpm uses more fuel than coasting in gear with revs about 1500.. Doesn’t make much sense to me but sure someone will be along in a minute to explain it to me in more detail, or possibly berate me for not understanding..

Too drunk to berate:), but this is how I see it*

Coasting downhill in neutral, engine running at 800 rpm:-
You have to provide fuel to turn the engine over at 800 rpm.

Coasting downhill in gear at 1500rpm:-
The engine could not even be switched on and gravity would still turn it over. Clever system can do magic and keep it ticking on f-all fuel.

*In theory, of course. If the electrocomputerygubbins is too simple all bets are off.

** Not too sure where the other option of just turning the engine off on downhills fits into this. Unless that was the original premis, which would just demonstrate I’m drunker than I thought.

Project: something missing from your post, surely?

I’ve been trying to make my dad believe this for years.

Its called Direct Fuel Cut off (DFCO)

He doesn’t believe me. even when the mpg gauge hits infinity when going down hill in gear with no throttle..

On the flip side, some high performance cars take teh opportunity of no spark during ‘dfco’ mode to cool the cyclinder walls with unburned fuel. – or so I believe, but I dont see how that doesn’t simply strip the ring – bore lubrication.

Most modern cars (anything this century mainly) will have fuel cut off under zero throttle if the engine is engaged to the wheels.
Also modern cars have much less friction and pumping losses than older engines, so the rate of engine braking is much less, and so its much better to coast down an incline in gear than out of gear.

One thing to remember though is that the cars ecu will start to fuel again if the engines rpm gets close to the tickover rpm (normal idle speed is 750rpm approx) this is so that the engine doesn’t stall. So when zero throttling I try to select a gear that comfortable keeps revs over 1000rpm.

It definately saves fuel, if you dont believe me then find a geek with an OBD2 lead and look at your cars ecu info as you drive.

If you car has a Medium wave radio you can hear the electronic injector pulses through that . Switch to MW and select a frequency thats not 5live .
The static clicking is the injector pulses . Go for a drive , if when you lift off compleatly and roll (in gear) up to red light the clicking stops you have ‘Overrun fuel cut off’.
When the car is nearly stationary dip the clutch and the magic fairies ( injectors) start to inject fuel again . BTW applies to most 1999- > petrol and derv’s.
If the hill is gentle and being in gear will slow the car down then select neutral and gravity will accelerate you, have some free speed .
If the hill is steeper and even being in gear you will still accelerate , then try to arrive at the top of the hill witha gap in front of you , leave the car in say 4th gear . Use as little braking as possible, and as the hill flattens out knock it out of gear and roll for 1/2 mile or so .
Gentle acceleration and short shifting are a betterererr way to save fuel IMO. ( combined with sub 60mph cruising)

down hill in gear in a modern ECU controlled car – yup the ECU will set the fuelling to 0.

Down hill in neutral or with the clutch in – yup the ECU will feed the engine enough fuel for it to idle as it would if you were stationary.

One question is:

is the engine braking drag have a net effect of more than the fuel used to idle the engine?

But the real world analysis is:

if you are in neutral down a hill will the car stay within the speed limit? If you select the correct gear for engine braking you may well find that you can stay pretty much bang on the speed limit without having to use any brakes at all. That way you are not losing any energy to the engine braking as you would need to scrub the energy off with the brakes anyway.

The only fuel saving would come from if you could just let the car build speed without any braking to conserve as much of the momentum for the flat/uphill bit to follow.

As for coasting in neutral or with the clutch in – no difference really. One will just make your foot tired but give you a faster response should you need to get back on the power. But I would strongly advise against any coasting in neutral as you are not in full control. Your steering response is best when there is some sort of power transfer through the wheels – this could be accelerating or retardation. Then there is also weight distribution which is affected by the same forces and at the extremes – ie none or maximum it will generally be detrimental – eg too much braking and the weight pitches forwards or accelerating hard and it pitches back and you lose grip.

Acceleration is preferable and racing drivers will feed power on gradually which gives the best response.

The best way to demonstrate this is find a good sweeping bend on a 40mph road. Go round it and with a bit of power applied to counter natural drag factors. Then go round it and apply a bit of heavy brake (not too much and make sure there is no one around you) and then go round it coasting (carefully). The car will feel completely unbalanced when coasting, the braking will make it a bit squirrelly and the gentle power will feel so much more in control.

I accept no responsibility for anyone crashing doing this but it also works with cycling when you powering through a bend. If you coast it you are not in as much control.

So when it comes to slow acceleration presumably there is a break even point?

It seems to take a lot more throttle to accelerate to 60 if for example you skip 4th and go straight to fifth from third.

Is it most efficient to just aim for minimum throttle and maintain about 2k revs mark regardless of gear. Or just try to drive everywhere in 5th?

by coasting, eg not having the car or vehicle in gear,there is no power to the wheels, thus swerving to avoid a pedestrain or animal who crosses in front of you will have no effect, just a loud reving of the engine.as your instinct and training makes you press tyhe accelaerator pedal

Not sure I unrest and your logic there ?

Are you saying that with the car out of gear you can’t steer (to swerve round an object)
Or that steering somehow causes the engine revs to increase ?

Coasting down the Col d’Aubisque uses more fuel than in gear and is more likely to result in cooked brakes. Back in the day when cars didn’t have power steering or brakes (or even column locks) lots of people coasted down long hills with the engine off (including my father).

I’ve never accepted that coasting is dangerous.

The implication seems to be that you need the ability to instantaneously accelerate in order to keep control of your vehicle, and keep it “safe” for all conditions.

The thing is (and I once had this argument with a police driver*) I can’t think of an single scenario where its safer to accelerate; for any given set of conditions, its always preferable to decelerate or brake.

There seemed to be a school of thought among “boy racers” that a reserve of speed is useful for getting you away from trouble; cobblers, of course- its always safer to slow down or stop.

(*His considered response, a couple of weeks after we debated this, was that there may be a theoretical “tired driver”, on a clear road who needs to get home and has maybe half an hour of useful driving in him, beyond which point he’s tuckered out. I feel that you would only burn away your concentration and endurance more quickly by taking your speed up.)

motivforz – Member
I reckon coasting can be done in a safe way on the roads, but it is illegal/against the highway code. I occasionally do it to save fuel, but keeping an eye on traffic behind, in front, road conditions etc.

Isn’t that what you do when not coasting?

Try this one. I was sat in the centre of an A road, waiting to turn right (oncoming traffic) I heard a skidding noise behind (was in a rag top car with the roof down) and glancing in the rear view mirror, saw a tipper lorry coming at speed towards me. I floored the car straight up the road, bouncing off the rev limiter. The tipper lorry turned onto it’s side, directly where I’d been waiting. It was therefore safer for me to be moving faster than to be going slower.

Which is a bit of a shame because other than the very, very rare occurrence, I completely agree with you!

by coasting, eg not having the car or vehicle in gear,there is no power to the wheels, thus swerving to avoid a pedestrain or animal who crosses in front of you will have no effect, just a loud reving of the engine.as your instinct and training makes you press tyhe accelaerator pedal

Your “instinct and training” may make you want to accelerate towards an obstacle in the road ?

Mine makes me steer round it, or brake before I hit it 😐

Use the time you would loose by coasting to blow your tyres up and empty the cack out of your boot for greater fuel efficiency. #viztoptip

I used to drive between work and my hotel most days, it was maybe 4 miles (should’ve walked or ridden, yes I know) and I got to know the road very well. It was overall down hill on the way back, and there was probably 6-700 yards of gentle downhill before the last roundabout. If I just lifted off, I’d slow down much too quickly and I’d have to keep on the accelerator for a lot longer to make it to the roundabout. It was more efficient (based on the trip computer score) to put it in neutral and coast, because you could roll the whole way without engine drag. The same effect could be had by tickling the accelerator really gently of course, but that was harder to do. That was in my diesel Passat.

Incidentally the Prius is great for coasting and rolling. If you lift off it’ll scrub speed slightly by recharging the battery – although it’s much less than in-gear engine braking; feels like an old style automatic lifting off. If you use light throttle it’ll just use the electric motor. But there’s a sweet spot – a tiny bit of throttle and the arrows on the energy flow meter disappear and you are just rolling. By far the most economical way to drive on rolling country roads is to gun it at the right moment out of corners or up hills or to get some speed up then use this coasting feature to roll through the rest of it. Hard to do when there’s other cars around though! A woman in Japan on deserted roads managed 115mpg over a tankful using this method to extremes ie speeding up to the limit then coasting almost to a stop.

Isn’t that what you do when not coasting?

Yes but adapting driving accordingly bearing in mind that I may be coasting, rather than just driving along. And checking even more frequently, looking further in advance et cetera.

Do we all have to explain everything explicitly to not be pulled up on the minutiae?

Do we all have to explain everything explicitly to not be pulled up on the minutiae?

Yes. I consider it a rather interesting technical challenge from a linguistic point of view. Requires some pretty adept writing, a bit like writing a legal contract whilst making it sound like an anecdote 🙂