Does anyone know where I can find a graph (or figures) showing fuel economy at different speeds.
I realise that it will vary for each vehicle but I wondered if there were some general principles that apply to all vehicles.
For example, I read that one reason for not increasing the speed limit on motorways to 80 mph is that fuel economy is much worse at 80 than at 70 mph.
Also, I have read that using air-conditioning reduces mpg. If this is the case how significant is it?
thanks,
Jon
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Aircon is not supposed to make more than 1 or 2 mpg difference.
Manufacturers no longer publish 'cruising' data as it was held not to give the public anything that was worth having.
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There is one school of thought that aircon is more fuel efficient than having the window / sunroof open to keep the car cool. While a road-car system will probably draw more, it was claimed that the aircon on the 206WRC sapped less than 3bhp.
As for the most fuel efficient way to drive, sit right behind a big lorry so you are in it's slipstream... ;)
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The effects of aircon are difficult to calculate depending on how much they are cooling the car but 3~6% is a resonably figure but it will depend.
The engine is most efficient hp per liter of fuel burned at its max torque figure (~2k for diesels) but ontop of that you wantto minimuse the speeed to minimise wind drag so although my Clio for example is most eficient burning fuel at 2k its doing 52mph at that sort of revs so if I slow down to 30 then although the engine is less efficient the drag is less so fuel ecomony is better.
Best advice is to drive as slow as possible in top gear for my dci 25mpg with the whole car ratting to death (courtestc car) was the most efficent no idea what but way way over 100mpg the fule computer wouldn't give any higer figure.
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I always understood 55 mph to be the most fuel efficient speed, generally. In highest gear, of course & still providing good progress.
Lots input regarding air con in the archives.
VB
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"Best advice is to drive as slow as possible in top gear for my dci 25mpg with the whole car ratting to death (courtestc car) was the most efficent no idea what but way way over 100mpg the fule computer wouldn't give any higer figure".
How absurd! Rattling at 25mph to save on fuel or turn off the aircon on a hot day like today to save a few pennies.
In the real world people like to get to places in reasonable time so 50-60mph is the slowest they'll be prepared to go on a clear motorway and most will do 70 since their work time and their spare time is worth more than the few pennies saved. If you really want to save money on fuel, either change the fuel type eg diesel or hybrids, or switch to public transport.
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Sorry if I gave the wrong idea I did it for 0.5 miles on a 30 limit emply road not to be cheap (I had a full tank) but because I wanted to see how far it would go I was getting 30mpg out of my Escort at the time and the Clio was getting 60 which had me amazed.
I have tried going slow in the Clio and at ~50 is the best speed but its slowing down and speeding up which causes the greatest drop not the speed itself usually from what I found.
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I have long suspected that 56mph figure was given as an attempt to get us all to slow down.
For example, when I used to do a lot of mixed environment driving in my old 2.8l Scorpio, I found I could hold 50-55mpg on the trip computer when driving at 80-90mph, I could not better that at 56mph or any other speed (aircon running or not!).
That resulted in an average high 30's mpg - not bad for a near 3 litre V6 automatic.
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55mpg from a 2.8-litre Scorpio - gerraway! If that was possible at 80mph I'm sure the makers would have told us all about it!
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I can only quote you what the instrument read and I had no reason to doubt it.
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"I can only quote you what the instrument read and I had no reason to doubt it" - yes you did, frostbite: an overall average of upper 30s. Parker's quoted range (which is taken from maker's published figures) was 21-35, and IMHO you would be lucky to get much higher than that. Anything in the 50s is pure imagination for a V6 petrol, unless maybe two cylinders were blanked off?
The only true consumption measure uses litres at the pump and miles from the odometer. Figures displayed by the ECU may be a good guide but can be a bit wild.
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For example, when I used to do a lot of mixed environment driving in my old 2.8l Scorpio, I found I could hold 50-55mpg on the trip computer when driving at 80-90mph, I could not better that at 56mph or any other speed (aircon running or not!).
I think you mean when driving at 90-80mph.
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I always understood 55 mph to be the most fuel efficient speed, generally. In highest gear, of course & still providing good progress.
This is a myth brought about by the fuel consumption figures which used to be quoted for constant 56MPH and constant 75MPH. Although fuel consumption is much better at 56 than it is at 75, it is better still if you go even slower. The best figures are usually achieved going as slow as the car will cleanly run in its highest gear. This is usually 30-40MPH.
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"The engine is most efficient hp per liter of fuel burned at its max torque figure"
Blimey - that's 4600 rpm in my Lexus IS200 according to the Lexus website! It all starts getting nice and revvy just after 4400-4500 rpm when the VVT kicks in. I always thought that this was the point where the engine said "Feed me, feed me now!" to the fuel tank...
As a related aside, at 82.1 p/litre round here, I'm seriously thinking about pumping my mountain bike's tyres up and starting that exercise regime that I've been promising myself for a few years now.
All the best,
CM
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Thing is its very "touchy" at max torque so difficult to often drive at a fixed throttle position and revs.
Also you don't use full throttle at max torque say 30% but at 30% throttle I think max torque is different.
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Manufacturers used to quote consumption at 56 and 75mph (=90 and 120 kph), and the difference was usually of the order of 20-30%. Largely because of increasing air drag.
Air-con needs a few BHP to operate, so it affects a small motor more than a big'un.
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Manufacturers used to quote consumption at 56 and 75mph (=90 and 120 kph), and the difference was usually of the order of 20-30%. Largely because of increasing air drag.
AndrewT,
I thought those mpg figures were measured on a 'rolling road' and thus air drag was not a factor - but stand to be corrected on that. What is without doubt is they were completely unrepresentative of achievable mpg under normal conditions.
Some manufacturers used to select the gear ratios and engine characteristics to maximise economy at 90/120kph; often at the expense of 'driveability'.
Older Mercedes always had graphs in the owner's handbook plotting consumption against speed.
C
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I think too many factors involved for anyone to give an accurate consuption figure on any motor.And as for what you said about rolling road.I fully agree.
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I bow to superior knowledge, Cardew, but I assumed they were done on a test track with precisely-tuned cars to get the best result. You may be right tho.
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I agree with AndrewT, there is no way that a 2.8 petrol Ford Scorpio will average 50 to 55 mpg at 85/90 mph. It takes a modern 2 litre common rail diesel engine all its time to achieve those sort of figures.!!!!
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Someone, very probably on this forum, once argued convincingly that the most economical speed for any vehicle was when the engine's torque was at a peak.
The argument was that that was the speed when the smallest movement of the throttle produced the most effect, so a constant speed could be maintained with the throttle position at its steadiest. As is I think well known, the steadiest throttle uses the least fuel.
56 mph in top gear quite probably represents peak torque in many common cars.
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Aerodynamic drag is a function of velocity squared, drag coefficient and frontal area. Above 50 mph it increaes rapidly - for example the wind drag at 80mph is about double what it is at 60 mph.
Another way to look at this is to compare top speeds of cars. A 150 bhp saloon car might have a top speed of 130 mph. To increase this to 160 mph you could easily need another 100 bhp of power.
Peak torque does correspond to maximum engine efficiency but won't necesarily give best fuel economy particularly for multi valve petrol engines that peak at high revs.
Diesels at very low speed give incredibly good fuel economy because they maintain high engine efficiency at low revs when aerodynamic drag is low. Petrol engines are not as good in this respect because the engine is throttled (effectively running with a very low effective compression ratio) so engine efficiency is very poor, but overall you still gaina little in MPG because the drag is low. Sorry to ramble on
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Does MPG increase as a car's engine is run-in? My Ford Galaxy TDi seemed to get more economical as it reached 36,000 miles, consistently hitting 40 mpg when it previously was getting 36 to 38 mpg on the same 30 mile motorway run.
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Most definitely. Any engine designer will tell you that the correct rounded profile for the piston rings needed for low oil consumption and friction isn't produced at manufacture. All new engines need to be run in at fluctuating speeds and loads so that varying amounts of 'tilting' of the piston can occur which controls the wear process needed to optimise the ring contact surfaces. Special coatings are now applied that assist this process. Bearings also cause less drag after running in as high spots are removed and surfaces smoothed out.
I don't know about vw diesels but in my experience peugeot diesels seem to get more lively and economical with mileage as they 'loosen up'. My 205 diesels now done 120,000 and it's running better than ever.
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A colleague who had a Galaxy TDI reported much the same thing: after about 30,000 miles, the fuel economy improved significantly.
My Mondeo TDDI just passed 40,000 miles, and is now getting 50mpg instead of the 46mpg it produced at around 30,000 miles, without any change in my route or driving style.
It's obviously just run in :-)
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In theory, it should be the speed in top gear at the rpm where maximum torque is produced, but In real-life tests I have done, 30-40mph yields the highest economy.
This is mainly because drag, in the form of wind resistance, friction in the powertrain and drivetrain all increase exponentially with speed.
Many people exclaim that their cars are most economical at 56mph, this is because mpg figures used to come in three figures, one of which was at 80kph (56mph) this was always the best figure and was misinterpreted by drivers as unique to their vehicle or some universal maximum efficiency constant.
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>>one of which was at 80kph (56mph)
Although many manufacturers would tune there cars so that the consumption was best at 56mph, frequently causing a flat spot in its performance.
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My understanding is that running an engine at or around the rpm producing peak torque will give maximum efficiency, where efficiency is a measure of power output per unit of fuel consumed.
This is probably the speed you would run an engine if it was being used in a generator or whatever.
However, most of us motorists measure economy in terms of distance travelled per unit of fuel consumed, which is not the same.
Ed.
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I read somewhere, (and it could have been HJ that said it?), that the best way to get good economy and still drive sensibly is to keep the revs in the 2k to 3k band.
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As I've posted several times now, for the past 2 months I have driven totally different to the previous 11 years of my driving life. At all times I have been driving to the road conditions, but when I have had the chance I have been changing gear at max torque, which happens to be 3300 rpm, and I've stuck around that figure in top gear when on motorways, which corresponds to 70mph. I am now getting on average an extra 110 miles out of the tank. Doesn't make that much more difference if I trundle about at 30/40, but I guess that's because I do that on A roads, with lots of stopping and starting for lights and junctions.
Coincidence that VW have geared the car to be at max torque at 70mph? I think not.
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Just a thought: by changing up at max torque, surely you are forcing the engine to work at quite a low point on the torque curve after the gear change. Would it not be more sensible to change up at maybe 1000rpm higher than max torque, thereby meaning that you start the next gear just below max torque.
While accelerating this would averagely mean you were closer to max torque for more of the time. Then when settling down to a cruising speed, stay at 3300rpm.
Does this make sense? Not explaining it very well, but basically I think it's better to keep the torque as high as possible overall, rather than just as a peak figure.
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Will give it a go.
Trying to achive maximum efficiency has made driving quite fun over the past couple of months.
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Some "ballpark" estimates for (petrol) family car:-
50mph (normally being min. viable for 5th gear) - 60mpg.
60mph - 50mpg. 70mph - 40mpg. 80mph - 30mpg.
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Some "ballpark" estimates for (petrol) family car:- 50mph (normally being min. viable for 5th gear) - 60mpg. 60mph - 50mpg. 70mph - 40mpg. 80mph - 30mpg.
I reckon you're not that far out. I've been shocked recently by the improvement in fuel consumption by driving my 1999Mondeo 1.6 at 70-75 on motorways rather than my previously habitual 85ish. I used to consistently average 35-37mpg from a tank, but I'm now up to 42! For 20-25k miles per year this is pretty significant.
Andy(surprised)
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I've been shocked recently by the improvement in fuel consumption by driving my >> 1999Mondeo 1.6 at 70-75 on motorways rather than my previously habitual 85ish
Andy, drop it to 60ish and you'll be even more surprised. I'm getting an extra 20mpg now.
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Yep your correct you want to miximise your time in the torque. You need your car rolling rodded to find where this is though Diesels torque is usually max at about 2.2k rather than the belived 2k same with max power 3.5k is often the max point 4k doesn't loose much power though and so for max accelation you'd need to dive uptoi 4k so when you chnage up you hit about 3k.
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Coincidence that VW have geared the car to be at max torque at 70mph? I think not.
Do VW alter the gearing of their cars for the different speed limits in each of their markets?
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From what I can remember from School. Air friction varies with the square of the speed you are travelling at. e.g. if you start at 30mph and move to 60mph, your air friction will multiply by four. Obviously the rate at which you accelerate between 30 and 60 has a big effect. Not to mention the characteristics of your engine!
Rednose
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Air friction is a direct function of speed whilst the flow is laminar.
However once the flow stops being laminar then higher order factors become more relavent, with third and often fifth order factors being dominant at higher speeds.
Thus at very speeds, unless the areodynamics are very good, a 10% speed increase could require about 70% more power.
As an example, the caterham 7, which has the areodynamics of a parachute, in 135bhp form will do 120mph, whereas the 230bhp version will only manage 150mph.
Its too late for me to look up the other models and top speeds, but it would be quite easy to use these to calculate what impact the higher order modes have.
--
I read often, only post occasionally
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Jon,
I have a Passat PDI 130 and had noticed that at a constant 60mph I could get 60mpg from the computer, this dropped to high 40's if I was doing 70mph but another 10mph sees a reliable return of about 50-55mpg. Whilst trying to nurse the car to 700 miles on one tank of fuel I checked the capacity section and then the driving for economy section of the manual (I can't believe I'm writing this) and it gave a rule that at 75% of maximum speed the car was using 50% less fuel than at maximum speed.
I have changed my driving style in the last 3 months and it has increased fuel efficiency by over 20% - simply looking further ahead to anticipate and look behind to prevent getting blocked in, it's not about driving slowly but at a relatively constant speeds. My Passat has 60k on the clock having just turned 2 yrs old, I own it and claim rate per mile for fuel - hence my anorak like interest in mpg recently. As for air-con, some threads have already said it, if you calculated 2-3mpg as a cost over the miles done in the tank versus getting hot and uncomfortable (whilst driving;-) I know I would pay. All my above returns were with climate control on all the time.
I hope that helps, if not sorry and good luck!
The sad git
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Right, I could explain to you in detail the principles of flight, jet engines and how aircraft manage to fly, and the way that temperature affects lift but I haven't a clue about piston engines. does the ambient heat affect the efficiency of an engine? The temperature is in the mid 80s today. Is my engine more or less efficient than in the winter?
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Low temperatures of the incoming air mean greater power, hence in high boost turbos which compress and heat the air, an intercooler is often used to cool the air again before admitting it into the cylinder.
Ian L.
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Any internal combusion engine will be at it's most efficient on on colder days (more so if air pressure is higher).
Colder air is denser, so you get more oxygen into the cylinders which in turn means better combustion.
As regards false economies, driving down the M11 yesterday with temperatures in the 80's I saw several Vectra's/Mondeo's being driven at 70-75 with drivers window down (and presumably aircon off).
If anyone thinks this is more comfortable than having the window up and the a/c on, jeez you're strange and don't worry I won't take a lift.
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I asked the question in the name of my quest to get ever better mpg figures. got it up to 48.5mpg the other day, but that meant sticking in lane 1 with the lorries doing between 50 and 60 mph. so when it gets colder I might manage the 50mpg mark!
Old before my time!
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I am getting less mpg from my Ford Ka as it approaches 35k miles on the clock, bought from new. The difference is small ~2mpg. It might be due to more short journeys.
I consistently find that in Winter the mpg - measured from litres put in the tank and miles driven - drops to average 40mpg in Winter, with a rapid rise in mid spring, and average 47mpg in Summer. Then in mid to late Autumn there is another sudden drop, with worst consumption in mid-Winter. I tend to suspect the petrol has special additives in Winter to protect against cold weather, otherwise why the sudden drop in Autumn and the sudden rise in Spring?
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I consistently find that in Winter the mpg - measured from litres put in the tank and miles driven - drops to average 40mpg in Winter, with a rapid rise in mid spring, and average 47mpg in Summer. Then in mid to late Autumn there is another sudden drop, with worst consumption in mid-Winter. I tend to suspect the petrol has special additives in Winter to protect against cold weather, otherwise why the sudden drop in Autumn and the sudden rise in Spring?
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Fuel consumption over short runs will be worse in the Winter for two reasons. 1. The engine takes longer to warm up. 2. More electrical equipment in use, e.g. lights, wipers, heater motor.
Does the sudden change in mpg coincide with the clocks changing?
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