All depends, if the weight and incline is enough then i would imagine the further distance travelled by coasting would more than make up the fuel saved over the shorter distance travelled on overrun due to engine braking effects.

I doubt there's much in it, but is coasting out of gear actually legal? and that is a genuine question.

The other thing is that there may be more wear and tear in the clutch mechanism and gearbox by regularly selecting neutral then re-establishing gear drive, which might outweigh the minute fuel savings.

I don't actually coast as such, though my lorry does this automatically going into N and coasting for considerable distances using pre mapped road topography, on some lorries its called eco-roll or similar, and i'm still not sure of its legality, on my particular vehicle this system cannot be shut down by the driver so long as the vehicle can 'see' the GPS satellites.

You can make much better fuel savings by using friction to slow you down instead of brakes, so you lift off the throttle 500 yards before a raoundabut instead of on the throttle till the last second and then braking heavily, do this repeatedly and especially making use of terrain and timing junctions so you don't have to stop and your fuel usage will plummet.

Edited by gordonbennet on 05/09/2018 at 22:05

Coasting in neutral is just the same as sitting parked with the engine running. Totally pointless.

With the added risk of snapping the crankshaft and I wouldn't have thought a DMF would appreciate that sort of treatment either

As always, it depends.

If you're approaching a red light (or any other reason to stop) engine braking in order to induce fuel cut off will be the most fuel efficient way.

If you're going through a dip, coasting in neutral will allow you to maintain, or indeed, even increase road speed, making the journey up the other side "cheaper", staying in gear invoking the fuel cut off at this point will slow the car unnecessarily causing you to burn more fuel to maintain progress up the other side.

"Coasting in neutral is just the same as sitting parked with the engine running."

Obviously not true - in fact, an absurd idea.

If you coast in neutral you are covering distance while the engine runs, which must be more economical than running the engine while going nowhere..

The current high end VAG stuff with 48v electrics will 'coast', but i think it actually switches the engine off completely. TBH i'm not convinced there is much point in a normal car putting it in neutral, i think the amount of fuel, if any, you'd save would be tiny.

I do remember one time driving a Saab 96 V4 which had it's 'freewheel' system engaged. After the initial scare i got approaching a give way expecting some engine braking(!) i found it quite entertaining. Of course the slippery shape, tall (for the time) but narrow wheels made it a very useful way of increasing your overall MPG, especially on a long journey, by a noticable amount.

Coasting in gear above around 1500 RPM results in the fuelling being cut off, but the engine drag causes braking.

Coasting in neutral (which I frequently do) results in no engine drag, but consumes fuel at a tickover rate.

You pays yer money and takes yer choice!

Does coasting in neutral save fuel?

Yes. This has been argued many times since my original riposte to HJ's erroneous opinion.

www.honestjohn.co.uk/forum/post/index.htm?t=86509

It is indisputable that it takes more energy to keep an engine plus its ancillaries turning at any speed above idling speed. Arguing about deceleration fuel cut-offs and the resumption revs has no bearing on the matter.

But turning an engine over using the kinetic energy you've burned fuel to build up is also wasteful - unless you're in a situation when the reduction in speed is desirable. An engine idling for 10 seconds while you coast down hill is not going to cost you anywhere near as much fuel as having to make up the 5mph deficit caused by your engine braking prior to reaching the upward gradient - 5 mph of precious kinetic energy you needlessly burned by lifting the throttle, inducing engine braking, rather than letting the car coast.

Hopefully if we ever get to the camless internal combustion engine (electromagnetic or similar valve actuation), then they can program the fuel cut off to simultaneously open the valves during fuel cutout so you're not wasting energy compressing unfuelled air in the cylinders, obviously they'd have to be non-interference engine designs!

We can have an endless debate about which produces the most economic result but I long ago came to the conclusion that it was utterly pointless and trivial.

Without examining each car and case separately it is dangerous to come to generic conclusions. By coasting Is cooling to the turbo adequate after a high speed run, are brakes and/or steering linked to the rotation of the engine, will engine restart easily when coasting mode ends, will disconnect between road speed and engine sensors corrupt fuelling systems, special issues related to hybrid and automatic vehicles. For the fractions aof a penny one may potentially save I would rather drive the vehicle as the manufacturer intended.

Not recommended by Highway Code rule 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.

Indeed. Very sound advice. Don't coast in neutral.

<< On my commute I stay in gear downhill and invariably have to keep my foot on the brake to stop myself going too fast. I’ve tried going out of gear for short periods, and honestly I cannot sense any engine braking in fifth gear. ... In the real world my hills have junctions at the bottom, and villages with schools and houses. So all this talk about hurtling down a hill,and back up the other side is hypothetical nonsense unless you are a dangerous maniac. >>

If you were driving a larger vehicle you might change down a gear or two to get more engine braking. In any case the question isn't necessarily about up and down hills, but perhaps anticipating a roundabout or other situation calling for slower speed.

I did some fuel economy runs in the '70s.

One was 'unobserved', in that an independant observer was not in the car. I'd rigged an ignition cut-off switch so I could coast in neutral without engaging the steering lock. My navigator was skilled in reading contour lines on OS maps and could inform me of approaching down gradients where I could coast. At one set of lights the staccato of starter motors of all us competitors was quite amusing. We had to average over 30MPH on a fifty mile course, mainly out of town. I got 93MPG out of a '73 Austin Maxi, beaten only by a 2CV at 97MPG!