Can you rely on your car’s mpg claims?

No, because in neutral, the car is using fuel to keep the engine ticking over, but in gear with your foot off the accelerator it is using no fuel at all.

If by ‘coasting’ you mean putting the car into neutral so that the engine is idling freely independent of road speed…..then yes, for modern cars this is less economical. The reason is that the car’s ‘electronic brain’ knows the mismatch between road speed and accelerator position and will simply stop supplying any significant amount of fuel to the engine.

Coasting used to be frowned on for a number of obvious safety reasons, including the lack of engine braking on a descent. However, modern cars are often so highly-geared in top gear, there is negligible engine braking in most circumstances anyway.

Coasting certainly can save fuel on a diesel engined car. The fuel consumption indicator makes this very clear. I don’t know about the legality but I only coast where I know the road and not when I have a passenger in the car. The argument is that you are not in full control of your car. That would certainly be the case if you were foolish enough to stop the engine.

With rising fuel prices, I would not be surprised if manufacturers introduce coasting as a feature of models with automatic or semi-automatic transmissions. There are safety features that could be built in to minimise the risks.

My father also taught me to put my foot on the clutch before starting a car engine, for safety and to disconnect the gearbox and put less load on the starter. I knew I must never do this when there was an instructor or examiner around. When I bought a new car recently the salesman warned me that I would have to remember to put my foot on the clutch when starting the engine. I said that I probably would not have a problem, having done this since 1968.

Interestingly, back in the 60’s when Saab still produced cars with 2 stroke engines, they actually fitted a freewheel mechanism which allowed the car to coast when you took your foot off the throttle. They did this to improve the awful economy associated with a 2 stroke and also to reduce the smoke from the exhaust.
The danger with coasting is the risk of the engine cutting out – rare perhaps, but it can happen, but when it does, you might lose the power assistance to the steering, making it very heavy – and you can lose the vacuum assistance to the brakes – reducing their stopping power.

I have read that other cars would coast too, though I don’t know the details.

Yes the engine could cut out when idling, but I can’t remember ever this happening to me, even when stationary. Maybe that’s why the Highway Code does not even mention this. If it did happen the dashboard warning lights would alert me to the problem. You are right about loss of power assistance to steering (one reason why I only coast on straight roads) but the vacuum reservoir provides power assistance for two or more applications of the brakes. I periodically do check the effectiveness of my brake servo and handbrake in deserted car parks to ensure they are working well.

I am certainly not promoting coasting but I doubt if I am more of a danger than those who allow themselves to run out of fuel on the road. A good habit that I learned from my father is to fill up promptly and so far I’ve not run out of fuel.

I believe it’s an urban myth that coasting saves fuel, and it’s dangerous. Just taking your foot off the accelerator while still in gear does save fuel though, as does a light touch on the accelerator of course.

If you acknowledge that taking your foot off the accelerator while in gear does save fuel, then is it not reasonable to think that you will save more fuel without the drag of the engine slowing down the vehicle?

I’m a cautious driver and far more concerned about the danger of overtaking. I have checked the Highway Code and there are warnings about the dangers of coasting and overtaking.

I wish I had not raised the matter since driving technique is not the subject we are supposed to be discussing.

Iin neutral, the car is using fuel to keep the engine ticking over, but in gear with your foot off the accelerator it is using no fuel at all.

Oh dear. I must just be imagining that I’m getting more miles per gallon when I coast. Let’s drop this discussion – please.

Agree with Nuke.. the mpg displayed on the onboard computer is 5-10% higher than the one I get by calculation on tank filling ( over several tanks full) , this is on a Skoda Yeti my old VW Golf mk4 figures didnt seem to show this difference.
The speedo also again does over-read by almost 10% .
However if it was simply a matter of the mileometer being wrong I wouldnt get a discrepancy between the displayed mpg and the calculated one !

Not sure how accurate sat-navs are especially at lower speeds and in town, the gps location can be a few metres out and would result in a more zig-zag path (and longer) than actually is driven. However I suppose with suitable smoothing this error could become negligible

Whatever methods are used to provide “realistic” mpg figures, individuals driving habits and journeys vary so much as to make them pretty academic.

We are a bit off-topic but the way we drive certainly has a big effect on fuel economy. I do the same at traffic lights but it does not seem to go down well in central London, where some drivers get upset if you start to slow down before they consider it necessary.

As far as I know, all manufacturers publish fuel economy figures in both mpg and litres/100 km in the UK. Many of us are keen on metrication but until miles disappear from road signs, most of us will continue to refer to mpg.

The graphics in Rob’s introduction make the differences clear, irrespective of what units are shown. If you really can’t cope with mpg, just assume that the manufacturers are being a little optimistic in their claims.

My car will display fuel economy figures as litres per km and litres per 100 km.

Neither miles per gallon nor litres/100 km makes any sense in a country that uses miles and litres but not gallons nor km.
As I said, why not miles per litre? That is how I work mine out.
Just how many here know what mpg actually means? How do you relate that to a gallon of petrol?

Which? has kindly provided a litre/gallon converter on their website:
http://www.which.co.uk/cars/driving/driver-tools/litres-to-gallons-converter/

Those of us who can remember fuel prices in shillings and pence might resort to a quaint system called mental arithmetic that was popular in the dark ages.

My car (Golf 1.6 D) was doing about 13.5 miles per litre but with the recent cold weather and short journeys it’s down to a depressing 11.5. The ‘combined’ figure given by Which? is 12.9 miles per litre, a bit more realistic than the manufacturer’s figure of 13.8 miles per litre.

The Which? converter uses an imprecise conversion factor of 4.55 instead of 4.546. This is a very small difference, so I’m being pedantic, but the website incorrectly says that 4546 litres is 999.12 gallons when the real figure is 1000.

I’m sure you are right, Malcolm. Part of the problem is that increased mpg is good whereas in litres/100 km, a lower figure is better. Maybe we should have gone for miles/litre, as suggested by brianac, when fuel sales moved from gallons to litres.

Just what “Age thing” is this Malcolm?
I still rememer petrol going up to 3/6 a gallon when I filled my motorbike, it does not mean I want either gallons nor £sd to return.
The fact is we no longer use either and have not done so for 30 years.
Live with it and get over it.

OK, thats fair enough. I too was educated for the most part in Imperial. I could not wait to get rid of its shackles and start using something a little more logical. I do not find it so easy to work in both units, I don’t want two incompatible units thrust on me such that I really understand niether. I do not like references to ‘my age group’, and ‘waiting for us to die out’ as being reasons for this country being held back. The gallon was removed from UK allowed primary units of measurment from 30 September 1995. It has been illegal for trade from that time. I see cars as trade, mpg displays are part of that trade. Peace.

I don’t think the EU would listen to Which?

I think the constant fuel consumption readings can be a bit misleading. They do indeed show your instantaneous fuel consumption but it doesn’t necessarily mean that minimizing it at all times will lead to better overall consumption on a complete journey. For example, when going downhill, the reading will show a very good consumption figure indeed – but all this means is that you are using up a different form of energy instead of petrol to maintain your speed. You are using up potential energy, in the form of altitude. Eventually, you will have to go uphill again, and fill up your lost potential energy reserve by using up more petrol (with very low mpg figures). Similarly, putting your right foot down to the floor will show a very poor instantaneous consumption figure but in fact it won’t result in a poor overall consumption because you’ve converted petrol into kinetic energy (speed) so haven’t in reality thrown energy out of the window. Indeed, by accelerating fast, you can afford to keep your maximum speed a little bit lower and still reach your destination at the same time, and by having a lower speed you can save fuel.

Brian, even some cars which aren’t ‘keyless start’ require the clutch to be depressed when starting. I had a 2003 Yaris where this process had to be followed and it wasn’t a keyless entry car.

I think this is due to what relative percentage of your journeys are in town, on single-carriageway extra-urban roads (whose speed limits are best for economy) and motorway. Some cars are better suited at higher speeds, others at lower speeds.

Clint, it doesn’t. I presume the EU tests should not be regarded as realistic of everyday performance, but are standardised tests to ensure all vehicles are tested on the same basis. Apart from being unrealistic, this also can lead to designers developing vehicles that perform well under the artificial test conditions, but not necessarily in real life. Best to use car reviews.

Malcolm, if the EU and Which tests neglected drag, they would be meaningless, for it would be impossible to compare mpg figures between two different cars, and that is exactly what those tests are designed for. For example, if they just measured engine and drivetrain efficiency then they would show that a square-shaped van had better economy at motorway cruising speed than a streamlined luxury car, when the reverse would be true.

Whilst a change in tyre size / width / make etc is likely to have a bearing on MPG, it might also be the fact that no two cars / engines are exactly the same in performance / MPG etc.

Malcolm, I think it is a misconception that full throttle wastes fuel. It is probably an over-simplification stated to discourage drivers from doing stupid things like accelerating towards red lights. From the Wikipedia page on Engine Efficiency: “Under part throttle conditions (i.e. when the throttle is less than fully open), the effective compression ratio is less than when the engine is operating at full throttle, due to the simple fact that the incoming fuel-air mixture is being restricted and cannot fill the chamber to full atmospheric pressure. The engine efficiency is less than when the engine is operating at full throttle.”

Clint – it all depends on whether you want maximum efficiency from your car or maximum economy. Using your theory, you could travel all day in 3rd gear at say 60mph to achieve “maximum efficiency”, but you would use much more fuel than doing the same trip at the same speed in top gear (say 5th or 6th).

I think most of us would choose top gear to improve the economy – this is after all the main objective of this subject.

AQ – Efficiency translates directly as economy, as it is defined as energy output as a percentage of energy input. (In this case, output energy is mechanical and input energy is chemical.) But this assumes that everything else is unchanged, so your example doesn’t hold. By saying everything else is unchnanged, I mean that under identical conditions (atmospheric pressure and temperature), and identical engine revs, an engine at half throttle is less efficient than an engine at full throttle. The engine at half throttle will be using a large proportion of its petrol just to keep running (the compressions take away a lot of power) whereas the engine at full throttle will be outputting something useful: acceleration. That’s the theory. In practical terms, it means that economical driving entails accelerating at full throttle to reach your desired final speed in the shortest possible time, and then reducing the throttle to maintain that speed for the rest of your journey in the highest possible gear. (I don’t think we are disagreeing, just interpreting the theory differently. And of course, most people would prioritise safety and smoothness before economy.)

Clint – whilst I am fully conversant with the laws of physics in relation to energy input, output and efficiency, you have drawn a direct correlation to economy with your argument – and all this is theoretical and bears no relation to real life driving, which was the original basis for this debate. Even the manufacturers’ MPG figures cannot be relied upon as to what is genuinely achieved on the road. If you believe you will get the best economy by flooring the throttle in each gear until you reach the speed you want, before easing back, you’re on your own – nobody else subscribes to such a practice for maximising fuel economy and nor would they recommend it. Safety and smoothness should, indeed, be high on everyone’s priorities – and are more likely to be achieved when driving with economy in mind, but if you insist that full throttle acceleration to the desired speed is best – then safety and smoothness, as well as economy have gone out of the window.

That’s not too bad. Avoid the Mini Countryman Cooper Diesel – Mini claim 64 mpg combined, but most people struggle to get more than mid-40s.

My brother has a Mercedes diesel. After his previous car was stolen, he replaced it with the exact same model (both were second-hand but with similar ages and mileages.) The second car has noticeably worse economy than the first one.

Gentlemen.

My vehicle, 3.2 ltr E series M/Benz ’04 saloon is not noted for its frugality. BUT, I think that on a motorway with 69.99mph set and a clear( ish) road ahead, the resulting 46.76 MPG speaks volumes for the auto engineers at M/B This is taken from the car’s computer readings which compare well with the Garmin Sat Nav’s trip stats, and are also borne out on the nifty iPhone ‘Fuel app’, which has returned an average consumption of 39 mpg since May 2012.

I brim the tank regularly as I don’t get many chances to fill up in the week. Mine is about 3mpg optomistic. I find the instant read out distracts me from the job of driving and it doesn’t really tell me much in the end. When the car goes into its ‘de-clog filter’ cycle the mpg drops for a while and then creeps back up. Its never below 40mpg and never above 45mpg. While on the subject, I find litres per kilometre meaningless since I never use it. If we converted I’d have to make the effort. I expect it would register in time.

I must go back to working out my fuel consumption, as I did in the days before fuel consumption indicators.. How often does a DPF filter go through a cleaning cycle? My car does indicate when this is being done.

Well, we all know that we are very unlikely to achieve the economy figures quoted by the car manufacturers, but it does not necessarily follow that if a certain model has a better economy figure than a particular rival, that drivers will also find that the same car still beats its rival in real life. Whilst a car promising 70mpg is very likely going to achieve better real life results than one promising 40mpg, quite frequently another rival promising 60mpg turns out to be more economical than the 70mpg car in real life driving. Although car manufacturers have made significant advances in improving fuel economy, the government prescribed tests result in a grave distortion between the publicised figures and what are really achieved. The figures might be as close as 10% worse on some models, but could be 30-40% worse on others. Interestingly, the greater amounts of difference are usually found on those models that promise the greatest economy, whilst conversely, the claimed fuel consumption of performance cars are usually much closer to what is achieved in real life – and sometimes even exceeded. So the differences probably come down to the manufacturers’ priorities model by model – where the primary aim is fuel economy, you must expect real consumption to be rather worse than the claim. Where fuel economy is lower priority for a particular car, then your own results are likely to be very much closer. As with anything that any government meddles with (of any persuasion) take the figures with a very large pinch of salt – best to rely on real figures.

“the claimed fuel consumption of performance cars are usually much closer to what is achieved in real life” – I’m a bit surprised by that. I would have thought that someone who bought a performance car would be more likely to drive it enthusiastically, with less consideration being given to fuel economy.

I don’t believe that these tests involve cold starts, though Which? does include a cold start in one of their tests. Cars are much less economical before the engine reaches the correct operating temperature, so it is hardly surprising that many of us do not achieve the fuel economy figures published for our vehicles.

Donna. It’s all down to driving style. The engine in your car was tested under laboratory conditions before it was ever fitted to your vehicle and produced some very fanciful figures so that the manufacturers could comply with the Law as pertaining to the sale of cars. They are merely a guide and as this conversation has shown, are no where near what would pass for the truth.

Your daily use may be to use the car for several short trips and your style of driving as you typify by admitting your age and status, will be quite gentle. Add the timing of your journeys…(School run both ways in heavy stop / start traffic,) and that will blow large holes in anyone’s attempt to eke out their car’s consumption.

Your local garage mechanic timed his trip ‘around the block’ to enjoy a light traffic time between 9.30 and Midday, plus I’ll bet my best boots that he warmed up the engine first, cleared out the boot and removed the roof rack. ( I obviously can’t know about the last two but if I was wanting to show how well a vehicle performed, they’d be high on the list) Correct tyre pressures, a well serviced engine, clean oil, the list just goes on and on.

Then he’d move out and get into top gear just as soon as traffic conditions allowed, and drive up to the speed limit 50, 60 , 70,mph briskly and time his approach to hazards to use the brakes as little as possible. He’d not be forced to sit stationary for long periods with the engine on, and all in all he’s easily shown you that a higher consumption figure is possible.

I bought a 6 cylinder 3.2 litre car just in time for the current fuel prices to go sky high, but using the methods which I have learned over 47 years of driving in 3 European countries, my car returns a regular 37mpg “round the houses” and a creditable 45 mpg on long trips via motorways.

Carl, achieving 55mpg on a run with an Octavia isn’t bad going, even if you have had better than this in the past. As discussed earlier on this topic, it is the Government which forces car manufacturers to publicise the results of the three sets of MPG tests, together with CO2 test results – all achieved in strictly controlled “laboratory” type conditions. The tests themselves are specified by the Government. The purpose was to “force” manufactureers to up their game on producing cars which would be more economical and less polluting, so had to be tested in identical conditions, which precludes real roads because of all the variables that would affect the results. The manufacturers have made significant progress in improving economy and reducing CO2 emitions, but have also exploited the potential to tweak various techniques to improve the figures. Apart from increasingly more efficient engines, lowering the suspension a little, fitting “stop/start”, fitting narrower wheels, adding spoilers and other aerodynamic tweaks, electric power steering – are all used in various combinations on different models. However, the changes to gear ratios probably have the biggest influence, after engine improvements. Final gearing is getting taller and taller, producing some spectacular results in the official tests and they will continue to experiment to beat the oppositions’ results. This might look good in their advertising, but bears no relation to real life results. I know a few people who have bought “greenline”, “blue motion”, and other “eco” versions of cars and have found that they are using more fuel than before – quite simply because much of the time they are in too high a gear – fine for steady speeds, but in urban conditions, you have to use lower gears and resist just pushing on the throttle to keep up with the traffic. Few people seem to achieve the manufacturers’ figures and only by driving gently can you get close – no bad thing to try, but difficult to maintain in everyday driving conditions.

As for claiming that the vehicle is not fit for purpose because you cannot match their figures – well you already know how far you would get with that, especially more than 3 years after purchase – nice idea though.