Very interesting MPG experiment!

Rob Dog

Member
I recently read some of the threads on this site about a variety of fuel saving measures that people employ in a bid to go a bit further on each tank full. This is something that is particularly appropriate these days, when a typical tank of fuel is costing between £5 and £10 more than it did before the end of last year!

One of the methods that was being discussed was the ‘keep it in gear at all times’ method as opposed to the ‘put it in neutral and let it coast’ routine. I’ve previously always naturally, by habit if nothing else, let the car coast along a road or down a hill when I’m not accelerating, even though I’m aware that it’s probably a better driving technique to always keep it in gear, even when decelerating. The general consensus on this site seems to be that the ‘keep it in gear’ method is always more fuel efficient too. Someone wrote that it was only the old carburettor engines that might save a bit of fuel by coasting, as it’s the momentum in newer injection engines that actually keeps the revs up (or something like that!). Jeremy Clarkson has also, at least twice now, preached this to viewers of Top Gear (even ridiculously as an alternative to actually using the foot brake!).

So, I thought I’d make a determined effort to try the ‘keep it in gear’ driving method to see exactly how much fuel I’d save on a typical tanks full. My 1.4 TDi (75bhp) usually returns late 40s/early 50s in terms of MPG with a combination of town and motorway driving usually on each tank full. I was somewhat disappointed however, to say the least, when my first full tank of juice returned only 43.92 MPG.
WTF(!) was my first reaction!

I then decided to hell with this ‘leave it in gear’ business and went back to full on coasting in neutral whenever and wherever I could. On a personal note, I don’t think it’s any less safe to drive this way, but I know others will argue against this no matter what I say!

After refilling this time, I was pleased my little motor had returned a more sensible figure of 51.70 MPG using the same driving style. But, in the name of science, I decided to give this experiment another go. So again, it was a tank full of keeping it in gear driving, followed by another tank full of coasting. The MPGs were 46.96 and 49.26 respectively.

My driving style over the four full tanks of diesel has been deliberately consistent, although if anything, for the last tank full of coasting driving (the tank full that returned 49.36 MPG), my driving has noticeably been a little more rushed due to being a bit more stressed recently- but I digress!

Of course, there will always be other minor factors that will affect MPG which my experiment has not taken account of, BUT, and this is a big BUT, you can see for yourself, that my ‘leave it in neutral and coast’ method seems to have returned a better MPG on both occasions. OK, so I should really conduct this experiment over a full year or so to get a better understanding of exactly which method saves more fuel, however I am more than satisfied with the results that I have seen so far, and will probably continue to use my existing ‘coast in neutral’ driving style, purely on what I have witnessed.

I think the ‘coast in neutral’ method works for several reasons:

• When you are in gear but not accelerating, the engine naturally wants to slow down, however if you are not ready to decelerate, then you will always have to apply more throttle, thus using more fuel. This is particularly appropriate on relatively flat roads where, when coasting, the momentum of the car will carry you along.

• When decelerating in gear, the engine’s RPM will always be greater. I’m no mechanical geek, but surely at a higher RPM, the engine will always burn more fuel than at tick-over. There will always need to be fuel in the combustion chamber?? Am I wrong?

• When coasting, and at speed, the air flow will be helping to cool the engine down easier than if the engine was still rotating at 2.5k or 3k RPM, perhaps even more. At tick-over, the engine is having to work less and will be therefore using less fuel.

There will be those who are reading this thread who will undoubtedly argue against the case for driving in neutral and coasting, and to those people, I don’t disagree with you in the slightest. All I am doing in presenting the facts - facts which I thought that a few others may be interested in hearing. If you don’t believe me, try it yourself. Give the experiment a go and then tell me I’m wrong. Remember to not alter your driving style though so that your results are fair and measured.

It will be interesting to see if anyone agrees with me! Happy experimenting…;)
 
When you are coasting in gear, you are using no fuel, but you are losing the energy required to spin the engine at the relevant revs.

When you are coasting in neutral, you are using (roughly) 0.5 litre/hour which is required to keep the engine spinning in idle. This is less than the fuel required to spin the engine at higher revs.

The key is to use the appropriate technique at the right time - for example, coming up to a stop, you should be engine braking/coasting in gear for as long as possible. Going down a gentle slope, you should be coasting in neutral since the slope keeps your speed stable. This latter technique can be used in something called P&G (Pulse and Glide) where power is supplied in brief bursts.

There is a 3rd variant of this, which is coasting in neutral with the engine off (actually in gear but with clutch on). I do NOT recommend this technique. I managed to get 72mpg out of a full tank on my Passat 1.9TDi when I was experimenting with it, but I destroyed my clutch in the process. In addition to this, on the A2 your steering lock may come on if you move your key too far (not relevant for the Passat), which as you can imagine is something you never want to happen. Do NOT try it!

See sections II, III and IV from the following article: (it's written from a hybrid viewpoint but some of the concepts are valid for TDis)
http://www.cleanmpg.com/forums/showthread.php?t=1510
 
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I employ both techniques depending on the situation...

If I'm cruising on a motorway and notice traffic slowing ahead, or if I'm about to take the next exit and need to lose speed, or if I'm coming up to traffic lights, etc, I always engine brake. The sooner you take your foot off the accelerator, the further you'll travel without using any fuel. I'll engine brake in 5th, then 4th, then 3rd, etc and only use the foot brake right at the end.
I'm sure it's not news to anyone that using the foot brake effectively uses fuel. Braking causes the pads to get hot. The energy used to warm the brakes comes from the kinetic energy of the vehicle which in turn came from burning fuel.

If I'm driving on an A-road and there's a dip in the road which then goes up a hill, I'll often coast. If I were to engine brake on the way down the hill, I'd be using no fuel over that distance, but I'd also lose momentum and then have to burn some fuel to get up the hill on the other side of the dip.
If you coast while going downhill, you may be using 0.5L/hour for a brief period, burning fuel to keep the engine ticking over. But you won't lose momentum through engine braking and will roll a further distance up the hill on the opposite side of the dip.

Cheers,

Tom
 
pretty much what the above posts said. My friend who did diesel engine testing for ford told me the ECU can indeed cut fuel if car is in-gear coasting. But if you're in gear, you need a lot of momentum and/or gravity to keep the engine ticking against drive train lost. So it depends how long can you sustain that. On a flat road, it is never long enough. On a long down hill slope, you can use no fuel in-gear coasting.

Add to that, air resistance is proportional to the square of velocity. In-gear coasting needs higher momentum working against the drivetrain lost, and so you're likely facing higher resistance too. Going neutral at 30 or 50mph has much lower air resistance than 70 and the car can travel a lot further.

I too use both techniques, and I'm familiar with how far I can go in-gear and time it accordingly. One thing is though, engine braking by down shifting sounds good, until your clutch is worn quicker and needs to be replaced ;-) I know, because I over do it.

Lastly, definitely DO NOT try switching the engine off. Steering lock aside, servo brake stops working too and you know that's very bad!
 
Coasting out of gear is, of course, technically illegal as you have not gpt full control of the machine...........................................................
 
Coast or not

Hi Rob, I have come to the same conclusion to you. Because my usual passenger gets really annoyed by me coasting out of gear I have done several tank to tank comparisons on my 1.4 SE and previous cars, Pug 1007 hdi (didnt keep it long yuck) Yaris 1.5 Tsport, great, drove like a diesel, + 45mpg but noisy at 70 and £175 tax, Freelander TD4 van, comfy but Sht build quality, A2 tdi blew up at 300ml (turbo) apparently fairly common, took 6 weeks to fix traded it in at 32000 2yrs cheapest car to run I ever owned 56mpg and serv due showing 500mls when sold. (prob because I was forever topping oil up after blow up) + AC Aceca bristol, Lotus elan S3 Gilbern etc etc. The A2 does 43.2-8 in gear and 46.25-89 out. I have had much the same results with all previous cars except the AC which if you "lost" a gear you could spend several hairy corners trying to find it again, and the Lotus which you needed to be in the right gear to blast out of the next bend. Oh such misspent fun in the late sixties early seventies when you could drive from London to devon and back for less than £20 Regards and sorry to drivel on Colin
 
I use both techniques & both are applicable.

However - If coasting you'll be be getting better mpg's if you coast faster ;)



I find the coasting technique adds a good 5mpg average to a 50 mile journey. A lot of Modern Hybids coast until you brake, eg Lexus/Toyota/Honda
 
I can't think of anything more dangerous than coasting out of gear. Keeping a car in gear when braking aids breaking too. If you need to make an emergency move to avoid the other idiots on the road you'll need to put it in gear first.
One of the major reasons I bought the car was for the economy but I'm not willing to endanger me or my passengers.
All for a gain of 2mpg. Not worth it.
 
I can't think of anything more dangerous than coasting out of gear. Keeping a car in gear when braking aids breaking too. If you need to make an emergency move to avoid the other idiots on the road you'll need to put it in gear first.
One of the major reasons I bought the car was for the economy but I'm not willing to endanger me or my passengers.
All for a gain of 2mpg. Not worth it.

Well said that man. I couldn't agree more, personally.

Coasting = partial control = accident waiting to happen.
 
“If you need to make an emergency move to avoid the other idiots on the road you'll need to put it in gear first”

Just curious, but what type of emergency maneuver requires instantaneous acceleration? I can't personally think of any bar perhaps motorway instances in which case I’m never in neutral anyway.

Also a long time ago I was led to believe that engine breaking accelerated engine wear. Perhaps it’s no longer the case? regardless i've tended to use only the brakes since.
 
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Have to admit there has been times when I've had to accelerate to move out of someones way.. rather have full control of my car whether I have to brake or accelerate...
 
for smoothness' sake, I tend to blip the throttle on the way down through the 'box and I am still on the first pads at 60k miles.
Brakes only puts excessive wear on them and engine only puts excessive wear on the clutch. Blipping helps but also increases consumption. Your call ;)

Bret
 
Coasting was frowned upon when cars were made with wooden brakes & engine braking accounted for a vast percentage of stopping ability. That is no longer the case but the legacy of all that still sticks. By coasting your car it will not cut out, the PAS wont stop working, the brakes wont stop working, the DSC wont stop working.

A £60k Lexus Hybrid will kill the engine & coast at 80mph if you gently decelerate. Any modern car the engine is off anyway under decel anyway! its just rotating, on diesels they now have a throttle butterfly to reduce pumping losses which reduces the engine braking to help overall efficiency on the drive cycle tests.

Every European car sold with stop start now requires you to be out of gear when stationary (auto's excepted) so you better all get used to knocking it into neutral at the lights & using the clutch pedal to restart the car.


The coasting is dangersous thing is something from half century ago - its not applicable in this day & and age (unless you still drive a jurassic)
 
When i first mentioned this a few years ago it was more for the approch to lights roundabouts etc....., rather than the long down hill road because yes you will have to get back on the throttle before you get to the end. I don't have DIS and haven't had it on any car i've owned but at work i drive cars that do and one day on road test i had it set to MPG's and noticed when you coast the mpg's go up to 8/900 mpg's but when you leave it in gear it goes to ---- so my question at the time was is --- more than 900 mpg's.

The answer was yes because you are not using any fuel, where 900mpg's is still using fuel to turn the engine

Phil
 
I have seen the same myself on a car with DIS. I personally agree that --- is greater than 900 mpg.

Who says Lexus/Toyota are getting it right? The are in the Guinness Book of Records for the highest number of recalls!
 
A lot of modern cars drive by wire and I would imagine if they do coast when they are not accelerating, as soon as the person hits the accelerator the car will automatically go back into gear. Also, wouldn't most Lexus' be automatics with cruise control?
 
For the safety argument, I wouldn't say it is totally wrong to say it is dangerous when you're out of gear, you're in partial control afterall.

But in practice, if you're in-gear coasting to save fuel, you'll be in 5th. Even with a remapped tdi 90, I would like to know how much 'emergency actions' you can do with the throttle while in 5th? If I'm coasting in 5th gear, slowing to 40 or even 30mph, the throttle response is next to useless, any action involving acceleration has to be in a lower gear anyway. Besides, when I coast out-of-gear, I have my clutch depressed and engaged in whichever gear is suitable for the speed (not entirely actually neutral). My emergency action is way faster than having to change from 5th to 3rd.

Engine breaking wears the clutch, (I sometimes heel-toe to match the rev but the electronic throttle isn't very responsive) the savings in fuel and brake is lost in the expensive labor to change the clutch. Consider this, a Mintex brake box is only £60-ish and you can DIY. Clutch is a lot more than that!
 
My last clutch lasted over 150,000 mile. Think I'll keep decelerating in gear as I can't afford a Lexus.
 
Here's the best way to improve MPG, reduce wear and tear on the car and reduce overall running costs.

Walk, don't drive.

Thank you and good night :)


Some (not all) modern hybrids do now "sail" at higher speeds - coming off throttle kills the engine and it "sails" (coasts using old language), and the electric motor spins the flywheel at a pre-matched RPM to enable the engine to re-engage drive as seamlessly as possible. Of course with a hybrid the car's sub-systems are kept running when in "sail mode" anyway by design so you don't lose hydraulic/electric assistance.

Hybrids will start getting interesting when we see diesel-electric range extenders which use all this technology, like VAG are developing, albeit in different ways at VW (conventional reciprocating) and Audi (rotary).
 
My last clutch lasted over 150,000 mile. Think I'll keep decelerating in gear as I can't afford a Lexus.

Either you don't use the clutch much or I completely overkilled it. I changed clutch at 46000 miles, the garage said there was absolutely nothing left of the plate! I drive the relatively torqueless 1.4 petrol, but I used to engine brake all the time hoping to save fuel and pads. And that was my reward, a clutch replacement bill.
 
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