Just seen a piece on how your speed effects your fuel economy and t doesn;t seem correct to me (unless my maths are completely wrong)
tinyurl.com/37fqzu {Link to www.northlan.gov.uk shortened to restore page width to normal}
Scroll to bottom.
They are saying that 50MPH is the most economical speed for the average car, travelling at 60MPH increases consumption by 21%, 70MPH +35%, 80MPH +42%, 90MPH +48%. I know these are averages but if I try to put some meat on the bones and use my car (1.8 TDCI S-Max) which gets around 50MPG at 60MPH:
50MPH - 60.5MPG
60MPH - 50MPG
70MPH - 44.8MPG
80MPH - 42.6MPG
90MPH - 40.9MPG
Now to me the 50MPH and 70MPH seems reasonable but the 80MPH and 90MPH figures seem way off, I would get considerably lower MPG's that they estimate.
Any opinions on the matter or have I mis-read it?
Edited by Dynamic Dave on 10/01/2008 at 13:43
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isnt the biggest problem the wind resistance as the car travels along in the air and gets increasingly worse above a particular speed?
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The "coefficient of drag" increases with the square of speed, and combined with rolling resistance (small) and frontal area is the important thing.
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The "coefficient of drag" increases with the square of speed
If I remember correctly, the coefficient of drag varies with the ride height and the angles of pitch and yaw, and the relationship is different for different vehicles. It's the actual drag force which increases proportional to the square of the speed.
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Ooops! brain/keyboard mis-triping on my part!
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I hae a Focus 2.0tdci and the figures are roughly what I'd expect to get. I'm not so sure about the lower end (heavy right foot!) but I tend to sit at 90 or so for a long motorway drive and average 41mpg.
I do notice a dramatic fall in economy going up to 90 from around 80. Can average 46-47 at 80 if I am trying.
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North Lanarksire, eh? They'll be advocating turning the heater off next - accompanied, no doubt, by a 'wee dram'..... :-)
Not sure about keeping tyres inflated to 'max load' pressures at all times :-(
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The Driving Standards Agency says that a car uses 30% more fuel at 70mph than it does at 60mph - different figures again.
Thinking back to James May in the Veyron - something like 250bhp to rach 150mph then another 750 to reach 250mph.
Edited by Andy P on 10/01/2008 at 14:14
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The Driving Standards Agency says that a car uses 30% more fuel at 70mph than it does at 60mph - different figures again. >>
30% more per what? gallons per mile, or per hour?
If you doubled the speed and doubled the fuel consumption, on one measure the car would be running no less efficiently, just going faster.
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I always thought this was a superb explanation of the amount of power needed for a given increase in speed. Certainly makes you think the first time you read it.
www.pumaracing.co.uk/TOPSPEED.htm
Cheers
DP
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DP - the article seems mainly concerned with top speed, not average fuel economy.
In the 'real world' cars have to accelerate, decelerate and go up hills. The way in which the right foot is used in these cases goes a long way in determining overall fuel consumption.
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The article in the original post seems to recommend accelerating gently to higher speeds and avoiding max revs. Maybe some of you engineer types can comment as I am sure I have read in the past that accelerating hard to cruise speed and then cruising is actually more economical.
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the article seems mainly concerned with top speed not average fuel economy.
True, but as the power comes from the fuel, it stands to reason that an exponential increase in power will result in an exponential increase in fuel consumption.
The factors affecting both power requirements and fuel requirements are exactly the same - rolling resistance and air resistance.
Cheers
DP
Edited by DP on 10/01/2008 at 14:38
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A good number of cars nowadays shows instant fuel economy.
While I don't trust them too much, but at least they give rough idea how much money I'm burning each moment.
And the figures will vary between cars.
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The factors affecting both power requirements and fuel requirements are exactly the same - rolling resistance and air resistance.
Under constant speed conditions, on a horizontal surface, that's true.
Masses require power to accelerate them, and power to do work against gravity when going uphill.
Few of us spend all our motoring lives travelling at a constant speed round a billiard table...!
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Lot of factors to account for, engine charateristics being one, if for instance a engine is at peak torque in top at 60mph it is likely that the car would be just as, or perhaps even more, economical at 60 than say 50.
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for instance a engine is at peak torque in top at 60mph it is likely that the car would be just as or perhaps even more economical at 60 than say 50.
That's what I used to think but I've read many items since that seem to disporve that.
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The engine might be running more efficiently at 60 mph but the extra on all the other factors would probably more than overcome that.
Trip computer in my car always shows higher at a constant 50mph (in SPECS roadworks) than it does at a constant 70 mph on an empty motorway, both in 6th gear.
70 mph is at the peak of the torque curve, 50 mph is way below peak torque.
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Who's going to break the news to SWMBO that the heated leather seats in her Focus are costing too much and she's not allowed to switch 'em on???
Edited by Waino on 10/01/2008 at 17:37
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The Honda does a miserable 31mpg on the motorway whether cruising at 65 or barreling it down an empty motorway at 85. Wretched thing. At petrol at £1/l I might as well chuck pound coins out of the window every 8 miles.
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Who's going to break the news to SWMBO that the heated leather seats in her Focus are costing too much and she's not allowed to switch 'em on???>>
Oops, I thought I'd added this to the thread on 'Effects of electrical items on fuel economy'. Don't tell me - another symptom of old age!
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I'd say on a fairly flat motorway these figures almost exactly fit my A4 2.0 TDI funnily enough.
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>>(unless my maths are completely wrong)
Yes, I think they are. I don't know where your base figure comes - it's not on the site AFAICS.
At 90mph they est. that consumption rises by 48% from consumption @ 50mph, yes?
So, (using your 'base' figure) consumption @ 90mph = 0.48 x 60.5 = 30mpg (approx)
Looks more in line with the math.
Next question.
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>>(unless my maths are completely wrong)
SNIPQUOTE for the person who didn't read the message about snipping and summarising when replying to posts!
Well the next question is obvious and is a question about your arithmatic?
60.5 is my 'base' figure that I have made an educated guess of but you are using my base figure and then multiplying by 0.48 to get 30MPG approx at 90MPH. So by your formula, at 80MPH I should get 25.4MPG as that is 0.42, at 70MPH I should get 21.2MPG as that is 0.35, etc. Diminishing fuel economy the slower you go?
48% refers to the increase in fuel consumption at 90MPH rather than 50MPH, not multiple.
I think my workings are correct.
Edited by Dynamic Dave on 11/01/2008 at 10:32
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Only accurate way is to properly measure the flow at the time over a variety of road conditions which beyond most of us however I was recently reading some old Motor road tests of a couple of cars I owned in the past and not sure correct way to working out the increase in fuel consumption/speed but I think the percentages are quite different from the OP's, mind you the cars are quite different as well!
Bristol 400 test dated 1948.Basically a pre war BMW 326 4 seater saloon fitted with 328 engine (6-cyl 12 valves and 18 pushrods )and made by the Bristol Aeroplane Company at Filton
Unladen weight 23.25 cwt or abt 1,160 k Tested weight 26.5 cwt or abt 1,325 k
1,971 cc
80 bhp @ 4,200 rpm
30/30 mph/mpg
40/31
50/30.5
60/27.5
70/25.3
80/22
90/17
Overall test ave 21.4 mpg over 1,009 miles
Price in 1948 £2,723.14s.6d including purchase tax! (Paid £90 for mine in mid 60's)
AC Ace-Bristol test dated 1959.Open 2 seater with a more developed 400 engine
Unladen weight 16.5 cwt or abt 825 kilos Tested weight 20.5 cwt or abt 1,025 k
1,971 cc
125 bhp @ 6,000 rpm
30/31 mph/mpg
40/30
50/29.5
60/26
70/23.5
80/22
90/20
100/18.5
Overall test ave 21.8 mpg over 1,765 miles
Price in 1959 £2,094.18s.2d (Paid I think about £750 in mid 60's and wish I still owned it!)
No torque figures quoted, they seem more concerned with mph/piston speed per ft/minute which I think something to do with the long stroke engines generally used and both cars geared around 20 mph/1000 rpm (4 speeds). Seems quite a flat consumption curve starting at a very high point however remarkably similar figures considering difference in power output, weights and style of both cars. I seem to remember both of these cars did around the low 20's (The Ace did abt 12 when I used to race it a bit) .
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Sorry, a900ss. The figure should have been the complement: 52% (100- 48) - I lazily did the sum in my head & put down the 'about half' figure & 48% instead of 52% - but the 90mph figure was nearly correct.
Should have been:
@ 90mph: (100 - 48)/100 x 60.5 : 31.4 mpg (my original was 30mpg)
@ 80mpn (100- 42)/100 x 60.5 : 35.1
@70mph (100 - 35)/100 x 60.5 : 39.3
@60mph (100 - 21)/100 x 60.5 : 47.8
@50mph (100 - 0)/100 x 60.5 : 60.5
Your calc. ,
@90MPH 40.9MPG would equal a % increase in consumption of: ~33%
I understand the '48%' figure to mean that the 60.5 mpg figure decreases by 48%
i.e. consumption nearly doubles - which would fit your actual observations.
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Those figures do make more sense. Thanks.
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It depends entirely on the weight and aerodynamics of the vehicle and the load on the engine, acting in conjunction with the weather conditions around.
Driving into a strong headwind of say 20mph will have the same effect as cruising 20mph faster - the equivalent airspeed is the same, likewise if you have a 20mph tail wind then cruising at 70 uses little more fuel than you'd expect to use cruising at 50 in still conditions. It's not going to be exactly like this because the rolling resistance is high at 70 than 50, but it won't be too badly out. Likewise a heavier vehicle will use more energy overcoming rolling resistance at higher speeds too.
The less aerodynamic the vehicle, the more effect it will have. A Discovery is going to suffer far worse in these conditions, or benefit far more, than say a Corsa. In real terms (percentage gain/lost) the Discovery will show greater variation because its aerodynamics like a house brick have more impact on overall energy needed to move it than the Corsa. The Corsa might well post a greater improvement in MPG though, because +20% at 25mpg is 5mpg, +15% at 50mpg is 7.5mpg.
What does affect things some is the load on the engine. Most of the time our engines run barely loaded, we demand a small amount of power continuously where the engine is running comparatively inefficiently, but we ask for little power. Alternatively we are accelerating hard and using lots of power but the engine makes it pretty efficiently, so the difference is less extreme than you might expect given the number of kW of power you actually ask for. A smaller engine more suitably loaded in cruising will provide better mpg than a big engine lolloping, not by much because both still run relatively inefficiently and to provide responsiveness manufacturers gear the smaller engine lower, trading some of this gain off. This is however why turbocharged engines are pretty good when cruising because the turbo backs off the boost, loading the engine more but having the ability to increase the power should the driver wish it.
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The effect of rolling resistance is proportional to the speed but the effect of air resistance is proportional to the square of the speed.
An engine is most efficient (not the same as economical) at the revs where maximum torque is produced. The gearbox is most efficient (again not the same as economical) in the direct drive gear, if it has one.
Back in my youth, before official consumption figures, Motor and Autocar used to test and publish the steady speed fuel consumption as part of their full road test. In all cases the consumption increased (mpg reduced) as the speed increased but it's a "banana curve", different for every model, where the rate of increased consumption increases as the speed increases.
Automatic gearbox cars with torque converter lockup clutches can be different - if the cruising speed is below the lockup road speed, consumption will be high due to continuous use of the torque converter. A number of cars with 80kph (48mph) lockups have better consumption at 50mph, than 45.
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It really depends on a lot of factors - weather conditions, individual car gearing, tyre type, etc. Here's a fact which blows all these eco-types figures into the scrap bin where they belong.
At a steady 60 mph, my Stilo averages 41 mpg.
At " " 65 mph, it gets 38.5 mpg.
At " " 70 mph, it gets 40.5 mpg.
Not content with trusting the fuel consumption figures to the onboard computer, I've actually done 'real time' journeys at those speeds. Using the fuel used and fill-ups, the result was VERY similar. So proof indeed, that some cars are MORE efficent at higher speeds. The green lobby NEVER tell you that !
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Here's a fact which blows all these eco-types figures into the scrap bin where they belong.
At a steady 60 mph, my Stilo averages 41 mpg.
At " " 65 mph, it gets 38.5 mpg.
At " " 70 mph, it gets 40.5 mpg.
..'Using the fuel used and fill-ups' & 'onboard computer'..
How did you manage to measure the fuel used so precisely (even doing part fill-ups) so that you could discern a difference of (+-)5% fuel consumed by a 'brim-to-brim' calculation? (presumably over a large distance)
Indeed, the (+-)5% figure could easily be 'contained' in slightly more or less 'urban' driving or a small hold-up on one journey etc. It's also difficult to imagine, on our crowded roads, the opportunity to drive for the length of time needed at steady speed to make such measurements (statistically) meaningful.
If the figures are correct, the general principles of the inverse square law of wind resistance vs. speed can't generally be bucked - even if your particular car has some quirk of engine management/power delivery/aerodynamics.
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With ref to your comment on how I managed to sustain those speeds for a long enough period to be meaningful - it's quite simple really. I live in the north of Scotland and regularly travel in the Edinburgh or Glasgow direction. That means 150 mls of single/dual carriageway, followed by a 3 lane motorway. If you stay away from rush hour traffic (ie. a bit of planning), there's no reason to be sat in queues or up & down the box every 2 mins. But I do take your point about varying conditions (eg. wind) and also that my car may have a mapping quirk.
After several thousand miles, I know with certainty that my car is more fuel efficient at 70 mph than it is at 65 mph. It does drop off alarmingly after 80 mph though, as I found out on a German autobahn !
Edited by GranTurismo on 13/01/2008 at 07:41
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15 years ago I took the opportunity to do such a trial in Australia where, once you get away from civilsation, there are numerous roads both uninhabited and relatively flat. I fairly regularly drove a 250-mile each way run on such a road, always under cruise control, so the numbers don't include my right foot effect. The engine of my 3.8 litre Ford ran 1800 rpm at 60 mph, and a bit over 2000 rpm at 70 mph. I can't remember the exact fuel consumption numbers, but, on a brim-to-brim check, the greater engine efficiency at higher rpm overcame the increased drag at higher speed to give me a lower mpg.
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Doesn't surprise me at all Gary. I think that applies to quite a few cars, but the green lobby won't even acknowledge it. This is where the so-called govt advisers fall down, because they can't see past a set of figures produced in 'set conditions'. They bare very little resemblance to real life driving.
Edited by GranTurismo on 13/01/2008 at 08:22
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