If you want to subscribe to such paranoia then there must have been less traffic due to people using there cars less.
I can't say I noticed any difference, apart from diesel engined car prices rising (being sought after) and petrol engined cars doing the oposite (I bought a petrol powered car in Sept)
|
While there are motorists out there daft enough to equate fast driving with good driving, they will continue to "impress" the rest of us the only way they know how.
Tomorrow, I will use the M1 southbound between 4.30am and 7.00am. It will be relatively quiet, especially early on. In that time, dozens of cars will overtake me at speeds well in excess of 100mph, with some who will be doing 130mph plus. (You've probably guessed that I don't expect to see a single police car during that time.) I am absolutley sure that any driver using public roads in such a fashion would NEVER consider fuel economy.
Should you, kind reader, think that such behaviour is bound to present itself on a semi-deserted, wide carriageway, consider this. Later in the day I will return home via the M40. By 6.00pm, this is heaving. I will see hundreds of vehicles driving between 80mph and 100mph plus. Bumper to bumper. Brake lights everywhere. And my mind will (again) boggle at the sheer disregard the average motorist has for fuel economy, the environment, and the safety of themselves and others.
Yes, there are motorists out there who conciously drive with regard for fuel economy. Probably about 7 in total. Each time fuel prices rise, another driver may join the club.
|
|
|
|
It always seems to me that drivers grumble about paying £1/litre (=£4.50/gallon) for fuel (which has to be dug up in inhospitable parts of the globe, then expensively processed somewhere else) but willingly pay £2.50/pint (=£20/gallon) for beer, most of which falls regularly from the sky at no cost.
Moan ye not ...
|
Very funny AT, excellent...
:o}
|
Those who tut-tut about cars overtaking them as they mimse down the motorway at an indicated 70 or less should consider the huge amounts of fuel wasted on ordinary A roads by people who imagine economy to be strictly related to a car's gross road speed, rather than the way it is driven.
I have often banged on here about it: those numerous twozzers of all ages and sexes who actually APPLY THEIR BRAKES to prevent their road speed from rising above some theoretical silly limit on clear downhill stretches and then labour up the opposing slope with heavy throttle, people who don't understand that in real conditions a car driven at 45 mph will often use more fuel than it will at 55 or 60, and so on.
I wish people had to learn how cars really work before being allowed to drive them on the road. There wouldn't be any motorways, we would still have carburettors and contact breaker ignition and the roads would be gloriously empty. I would willingly give up the modern automobile just for the clear roads.
|
I second that!
My instant fuel economy display [assuming it is correct], display minimum consumption when I am driving at mostly constant speed, be it at 50 or 80 mph.
It's goes high when I'm accelerating and driving up a slope.
I don't see much economy improvement while driving between 60 & 80 mph. Over 90 mph, I do see that my car consumes fuel a noticeable amount more.
So, traffic permitting, I always try to stick between 70-80 mph on motorways to get best balance of time, economy and driving enthusiasm.
Edited by movilogo on 03/05/2009 at 15:34
|
I fail to see how anyone can get annoyed at someone overtaking them on a motorway, whatever speeds are involved. It doesn't affect you, the risk to you is negligible, so why get so bent out of shape about it? This isn't a moronic overtake on a blind B-road bend or hump bridge, which could take you with it, but a planned manoeuvre on a road specifically designed for it.
If they are doing 100+ mph, so what? It's their fuel and license.
Edited by DP on 03/05/2009 at 15:40
|
The risk is not negligible: whatever the situation on a truly clear motorway, driving much faster than other traffic is inherently dangerous because the faster vehicle may arrive too quickly for other drivers to register it. A driver on a motorway may have a clear view behind of, say, 200 metres. A car travelling faster by 40 mph (18 m/s) will cover that distance in 11 seconds and will be extremely close in much less. Even an attentive driver, checking behind every few seconds, can get a nasty "Where did that come from?" shock when the speeding car (black, German or - occasionally - otherwise) appears from what, moments before, was a clear road. I know; this happens to me and I reckon I'm pretty punctilious about mirrors. (I even clean them before I set off.)
This is where all the bleatings from the ABD and others that modern cars / roads / underwear are so much better than they used to be are shown up as the drivel they are. Newton's laws, as they affect vehicles, have not been repealed or amended, and vehicles are still controlled by the same human drivers with the same fallibly slow perceptions and reactions. So wilfully denying someone else the time and space to drive safely - whether by pulling out into their path, or by appearing behind them faster than they might reasonably expect - is both antisocial and dangerous, and therefore bad driving.
And, to return to M. Escargot's original topic, there do seem to be fewer cars on the roads these days, on the M40 at least. My typical driving time to work and my fuel economy have both improved since the start of the year. I suspect, though, that this is less to do with saving fuel than with fewer people having work to travel to.
|
So wilfully denying someone else the time and space to drive safely - whether by pulling out into their path, or by appearing behind them faster than they might reasonably expect - is both antisocial and dangerous, and therefore bad driving.
Sorry WDB but that is cobblers. If we all have to avoid appearing behind people 'faster than they might reasonably expect' everything will grind to a halt. And that's what you anti-speed wonks secretly want.
If they are in a car on the road they have the time and space to drive safely. Provided they look in their mirrors before changing lanes or turning off, they are driving safely. If they leap out of their seats in shock like the double-take brothers when a car overtakes them they belong in a psychiatric ward, not a driver's seat.
|
|
|
|
Its down to our poor driving skills and standards, how do the mainland europeans manage with their bits of derestricted motorway? I would like to think I use my mirrors more than most, but I agree that cars, and more likely motorbikes, can arrive behind you between even frequent mirror checks when travelling way above the speed limit. Its down to speed differentials.
Edited by Old Navy on 03/05/2009 at 16:53
|
how do the mainland europeans manage with their bits of derestricted motorway?
In my experience a lot of them drive as lousily as the British, in the same half-witted mimsing way.
But at least they aren't taken by surprise and filled with indignation when someone comes past them at a decent lick. Or if they are they have the sense to keep quiet about it.
|
But at least they aren't taken by surprise and filled with indignation when someone comes past them at a decent lick. Or if they are they have the sense to keep quiet about it.
Too many wannabe traffic cops and "road owners" in the UK.
|
|
It comes down to expectations (otherwise known as anticipation).
If you are on a 30mph limit road, as a driver, rider or pedestrian, you are likely to be taken unawares by a vehicle travelling at 40mph.
Ditto on a 70mph road by one doing 80 or so.
Without a limit you known that much faster traffic is a possibility so watch for and allow for it.
|
|
|
|
|
If they are doing 100+ mph, so what? It's their fuel and licence.
Some of it could have been someone else's fuel if the speed merchants hadn't burnt it first ...
|
|
|
|
My instant fuel economy display shows minimum consumption when I am driving at mostly constant speed, be it at 50 or 80 mph ...
21st-century cars are typically about 50% heavier than earlier models, which will mean that they need relatively more fuel to get up to speed, but once there they may need relatively less to keep rolling on the flat. But wind resistance increases (I think - NumberCruncher will confirm) with the square of speed, so consumption at 80 will always be more than at 60. Which itself explains why modern cars are mostly the same shape, because wind-tunnel testing should always lead to the same optimum result.
|
DP,
I don't give a monkey's about being overtaken as long as it is done safely and within the law of the land - my land and that of fellow road users.
I am not allowed to drive at over 70mph on motorways. Nor is anyone else.
At 100mph, a car is probably using a minimum of 30% more fuel than it would at 70mph. Pollution products would be increased similarly, if not at enhanced levels. This cannot be permissable in this day and age. Are we Neanderthals?
Opening this discussion out to others - and digressing slightly: What nobody ever seems to take into account regarding road speeds is that, for many people, the UK road system is a workplace. It is my firm view that such individuals should have the absolute right to be allowed to remain safe in that workplace. If that means everyone has to "mimse" around, then so be it.
|
Another damn amateur policeman...
How I despise self-appointed prefects. Not all that keen on official ones either.
Edited by Lud on 04/05/2009 at 02:03
|
|
|
I am not allowed to drive at over 70mph on motorways. Nor is anyone else.
I still don't understand why it matters so much to you if someone else does? They're the ones risking the wrath of the law, not you. It is up to the police to enforce the law, not you. If it makes you feel so much better, get the registration number, pull up in a safe place and report them.
At 100mph, a car is probably using a minimum of 30% more fuel than it would at 70mph..... This cannot be permissable in this day and age. Are we Neanderthals?
Oh, come on! If you were that concerned about pollutants, quite frankly you wouldn't be driving a car in the first place. In the general scheme of the overall environmental impact of the car you are using, the extra fuel used between 70 and 100 mph at a given moment in time is so small as to be pretty much irrelevant.
for many people the UK road system is a workplace. It is my firm view that such individuals should have the absolute right to be allowed to remain safe in that workplace. If that means everyone has to "mimse" around then so be it.
On stretches of road where people are working, a blanket speed restriction is imposed in the area, enforced by average speed SPECS units. I don't believe anyone has any objection to these at all. I certainly don't.
|
I used to drive a friend's 320 CDI S-Class on quite a few long trips - I averaged 35-36mpg without holding back.
My own 1.6 petrol VW Bora averages around 40-41mpg on a run and around 35mpg in urban areas.
I know which can be regarded as the greater engine achievement considering the weight of the vehicle involved; same applies to petrol equivalents from manufacturers such as MB and BMW.
|
the extra fuel used between 70 and 100 mph at a given moment in time is so small as to be pretty much irrelevant.
Sorry DP, that's not right.
At speed, the car is doing most work against aerodynamic drag. The power required to overcome aerodynamic drag rises with speed *cubed*. This means that the mass flow rate of fuel into the engine is proportional to speed cubed.
So,
(100/70)^3
ans =
2.9155
So, instantaneously, the mass flow rate of fuel into the engine is almost 3 times greater at 100mph when compared to that at 70mph. If the fuel stays at the same temperature, then this rate is also true in the sense of volume rate of fuel flow.
When you look in terms of mpg, because when you travel faster the journey takes less time, the relationship drops from being proportional to speed cubed to speed squared, so, in terms of mpg;
(100/70)^2
ans =
2.0408
mpg should be half at 100 what it is at 70.
What speed you choose to travel has a huge and significant bearing on fuel consumption, and along your choice of car, and your maintenance regime, is the most direct way you can influence how much fuel you use.
|
"mpg should be half at 100 what it is at 70."
As i`ve said before, you notice the effect on a `naked` bike - in particular with wide-swept bars and a sit up riding position.
60mph is a wisp of throttle and being caressed by airflow. 90 mph is virtually full throttle and the air trying to push you off the bike. You can cruise all day, sitting upright at 60, but at 90 you can`t keep it up for more than 20 minutes or so without being exhausted.
Take it to 100mph - full throttle sitting upright on a steep downhill and you risk lack of control of the bike due to sitting upright and literally hanging onto the bars - as the air feels almost solid and you have what feels like around 100lbs of pressure on your upper body.
I think it`s a tribute to modern car design that you often don`t experience the effect of massive increasing air pressure as speed goes up past 90mph. No buffeting or howling wind and the engine often smooths out at higher speed.
But the increased fuel consumption has to be there due to that massive pressure being applied on the cars front.
Edited by oilrag on 04/05/2009 at 14:23
|
Sorry DP, that's not right.
Sorry, NC, I was referring to the overall environmental impact of the car as a method of transport, not just fuel economy. I understand your (as usual) very clear explanation, and it makes sense.
However, if we're considering environmental impact, let's look at the whole thing instead of just picking fuel economy. I understand it is correct that energy consumption and emissions from a vehicle in use are pretty trivial compared to those required to build its various components (including raw material extraction / refinery / transportation), transport them to a factory, assemble and paint the car in the first place. It therefore stands to reason that the additional fuel used between 70 and 100 mph, although considerable as you say, is also pretty irrelevant in terms of the overall picture.
If I'm wrong, I'm happy to be corrected.
Edited by DP on 05/05/2009 at 12:43
|
>>terms of the overall picture.
There's the nub of the problem - who can be trusted to present the big picture?
Car companies and pressure groups like the SMMT will always downplay the energy and pollution caused in making and selling a car, while pro-motorist groups will do the very opposite.
|
So instantaneously the mass flow rate of fuel into the engine is almost 3 times greater at 100mph when compared to that at 70mph.
Of course, another problem is that the efficiency of the engine is likely to be related to power output, so it becomes difficult to theoretically derive the fuel consumption at various speeds. Fortunately, this quantity must have been actually measured for various cars. Maybe someone has come across reliable data on this?
|
Yes, typically, you get the best *specific* economy at about the engine speed for maximum torque, and between 2/3 and 3/4 of maximum engine load.
So, yes, changing engine speed will have an effect, but, compared with the cubed relationship of power with respect to speed, it's not too significant.
In the past, I've posted links to engine maps which show this relationship - the problem is that as most buyers are not sufficiently technically literate to understand torque and power curves, an engine performance map would blow their minds, and so, is very rarely available in the sales room.
|
In going from 70 to 100, unless you have very tall gearing, you're likely to be moving to a less efficient part of the map, making the increase in consumption worse than the simple theory which I posted above might suggest.
|
|
There are various people/circumstances where speeds in excess of 70MPH are allowed, even in unmarked cars. Where this is allowed, it is allowed for good reasons. You don't know where this applies and where it does not.
There is also the recurring problem of speedo errors meaning your 71MPH might be someone else's 65MPH.
Enforcement of road rules is best left to the professionals - for the above and other reasons, but mainly because enforcement of the rules can itself create danger.
Speed does have a major influence on MPG. But don't gild the lilly too much. The opportunities to go at 100MPH are limited by our overcrowded roads. Choice of car, driving style and maintenance can have a huge effect.
I recently changed from a medium sized car where I normally averaged about 38MPG to a (much) larger car - where I can easily average 48MPG. Last weekend I did a 300 mile round trip up the A1. By driving carefully (and using one heck of a lot of anticipation) - we managed 58MPG, whilst travelling at or just under the speed limit most of the way.
If you really want to improve overall MPG, then use and advocate anticipation. This includes anticipating faster cars and seeing if you can (gently) get out of their way. Even when they are driven by obvious loonies and speed freaks.
Edited by ajb1 on 04/05/2009 at 13:51
|
DP,
Others have dealt with fuel consumption issues, so allow me to get back to the off-thread issue of UK roads being a workplace.
I was actually thinking of people who DRIVE for a living. I believe that drivers of all types of vehicle are constantly endangered by the wilful negligence of others - in a way that is not possible in any other area of endeavour. The very least we can ALL do is respect posted speed limits at ALL times.
|
In my experience, some of the most dangerously driven vehicles on the road are piloted by those who drive for a living. A minority of HGV's and white vans are the two that spring to mind. As for what represents the biggest danger to me as a private motorist in a typical 1500kg car, I'd take another 1500kg car at 100 mph over 15,000+ kg of artic at any speed.
I've driven approximately 300,000 miles in the last ten years, and I genuinely cannot remember the last time I felt endangered by someone speeding. I can however recount many times I felt endangered by someone pulling out on me (the cause of my only accident since I started driving 20 yrs ago) tailgating, swerving across the road while remonstrating with their kids (the cause of SWMBO's only accident in her driving career), driving close to me in a car that clearly hasn't seen a mechanic's spanner since the warranty expired, driving while clearly inebriated in some way, or jabbering away oblivious to all and sundry on their mobile phone.
There are far more threatening and genuinely life endangering issues on the road than someone overtaking on the motorway at 100 mph.
Edited by DP on 05/05/2009 at 12:18
|
Agree with DP.
On the fuel consumption issue, I've found my MPG on daytime motorway journeys to be higher (i.e. less fuel used) cruising at 80 / 85 than at 70 / 75.
I believe this is due to less baulking etc at the higher cruising speed. At the lower speed, I find there's more braking / lane changing / accelerating because of mixing with the trucks etc in lanes 1 and 2.
|
I've found my MPG on daytime motorway journeys to be higher (i.e. less fuel used) cruising at 80 / 85 than at 70 / 75 ...
Sorry, Craig, but I don't see why the laws of physics should not apply to your car (driving) as they do to anyone else's. As explained in other threads, it unavoidably takes more energy to push your car through the air at 80 than at 70, other factors being equal. If you are quoting the readout from your car's computer, I suggest it has left some values out of its calculations.
|
>>>> I believe this is due to less baulking etc at the higher cruising speed. At the lower speed I find there's more braking / lane changing / accelerating because of mixing with the trucks etc in lanes 1 and 2.
This looks like a version of the BMW phenomenon recently exemplified in the BMW April Fool joke, in which the ubermensch intimidate other road users (untermensch) out of the way, thus making for smoother maintenance of law breaking speeds.
|
VAGs 6 speed gearboxes fitted to the TDis have a very tall (a bit too tall IMO) top gear, 2200 RPM equates to 80MPH and you're driving in the cars power sweetspot (max torque), drop down to 65 / 70 and to climb an incline you need plenty of pressure on the gas pedal to maintain speed in 6th (don't say change down a gear then because in reality hardly anyone does). You can check this out using the instantaneous MPG read out rather than the average of the journey.
|
Let me first state the law of physics :)
We know, power = force * velocity
So, the power required to move a car in velocity V is
P = (Cd*r*A*(V^2)/2 + m*m*g + m*g*sin(q))*V
where
Cd = coefficient of drag (usually 0.2 to 0.35)
r = air density (1.3 kg/m3)
A = frontal projected area of the car (~1.5 m2)
V = velocity of car (m/s)
m = coefficient of friction (~0.15 for rubber tyre on concrete road)
m = mass of car (kg)
g = acceleration due to gravity (9.8 m/s2)
q = gradient (or slope) of the road (radian) equals 0 on level ground
P = power required to move the car (W)
The law very clearly states that driving at 80 mph will need more power than driving at 70 mph
however
that does not mean fuel consumption at 80 mph will always be more than compared to 70 mph!
If fuel consumption curve is plotted against engine's RPM, it has an optimum range for least consumption (varies among engines).
Above or below that RPM, fuel consumption rises - so less consumption at higher speed perfectly possible (and instant fuel economy display does prove that, they can't be that wrong in all cars)
Also, I find it bit funny when people driving luxury cars advocate driving slow to conserve fuel! Would they not save much more money by buying a cheaper car instead?
|
+ m*m*g
This term is wrong.
>>If fuel consumption curve is plotted against engine's RPM, it has an optimum range for least consumption (varies among engines).
As I've explained in an answer above, yes, there is an effect, but, compared with the power required to overcome aerodynamic drag at high speeds, this isn't a strong enough effect - and at reasonably high speed, you're almost guaranteed to be moving away from the engine's "sweet spot"
|
Sorry, this is correct version.
I pasted here from Word and Greek letters became plain English!!
P = (Cd*r*A*(V^2)/2 + u*m*g + m*g*sin(q))*V
where
u = coefficient of friction (~0.15 for rubber tyre on concrete road)
Everything else is fine.
|
All I can tell you is that 5 years ownership of a 6-speed Passat PD130 showed that an 80 / 85 mph cruise was more economical than a 70mph cruise, both on the computer and at the pump.
I'm not claiming the Passat (and my current Mondeo) use progressively less fuel the faster I go. After 90mph consumption in the Passat rose markedly. But around 85 it was in the "sweet spot" mentioned above.
I understand and can read power, torque, BMEP and specific consumption curves, I know about aerodynamic drag etc etc, and I agree that in ideal conditions and on paper, higher speed should mean more fuel used per mile.
But I also know that real-world driving conditions (traffic etc), the specifics of a particular engine design, the way the car is driven and even differences between a "Wednesday" and a "Friday" car throw in factors that will make substantial differences between the 'ideal' and what's actually observed.
Otherwise we wouldn't be discussing how very few of us can achieve the official consumption figures in our cars.
Edited by craig-pd130 on 05/05/2009 at 20:44
|
criag-pd130: some people will never believe you.
But real world road consumption (not mimsing on the motorway) is often a bit surprising. I had a Lada 1200 that would never do better than 28mpg however carefully tuned and carefully driven. And A VW 411 that did 28 mpg at all speeds. And a Renault 18 GTX that wasn't hugely frugal at low speeds, although not bad, but would return 30mpg cruising anywhere between 80 and 100.
There's no way of working it out from the gearing, air resistance and so on. The proof of the pudding is in the eating. And as any fule kno, steady speed even pressing on is often more economical than shunting up and down among the hoi polloi at ostensibly more economidal speeds.
|
>>There's no way of working it out from the gearing, air resistance and so on
You can believe that if you like Lud.
|
Let us say: you can work it out of course allowing for all factors. But the answer you get will be for a constant speed on level ground in still air. So it won't be what you will get punting about the country.
You can make an educated guess from a car's specification as to its likely consumption under given conditions. But that's all it will be, and it could be wide of the mark.
Will that do sir?
:o}
|
It is definitely true that driving at a target 70 mph (and keeping lane discipline) requires more acceleration / deceleration in typical modern motorway traffic than driving at a target 85 mph . Both my cars are still more economical at 70 though, but probably not by the margin they might be.
Edited by DP on 05/05/2009 at 23:04
|
>>You can make an educated guess from a car's specification as to its likely consumption under given conditions. But that's all it will be, ...
Any major manufacturer will already have extremely good estimates of how the finished vehicle will perform in the standard emissions and fuel consumption tests, and the other performance figures way before any metal gets cut. It's the difference between engineering and guessing.
|
Any major manufacturer will already have extremely good estimates
Yes, yes, of course, calculated to the nth degree, double-checked and published as Manufacturer's Performance Figures. I wasn't suggesting that they (or you) were guessing.
But in real use there are quite wide variations in fuel consumption owing to the way people drive, terrain, traffic conditions and so on. It's the motorist, not the engineer, who has to make the educated guess. Those manufacturer's figures are usually part of the process.
Let's put it this way: a motorway eater with a 35mph per 1000 rpm top gear won't be ideal for delivering milk in the back streets. And so on.
Honestly NC, why do you misunderstand me so persistently?
|
>>Honestly NC, why do you misunderstand me so persistently?
It's how I naturally read and interpret your posts - I'm not trying deliberately to mis-read them, or twist them.
Honestly Lud, why do your posts antagonize me so persistently?
:-)
|
u*m*g
Sorry, not just the formula, but the concept is wrong. The tyre is rolling, not sliding, therefore, there's little to no work being done against friction.
That term simply should not be there - it has no place in a road load equation.
However, what should be there is a term for the rolling resistance of the tyres. In force terms, it's
m*g*(A+B*V)
Where A is the constant part of the tyre's rolling resistance, and B is the part which rises with speed, m is vehicle mass (kg), and g is the acceleratio due to gravity. All of this term would be multiplied by V again to express as power.
Typical values for A and B are;
A = 0.025
B = 0.0001
for speeds expressed in metres per second.
|
............ there's little to no work being done against friction.
........... the rolling resistance of the tyres.
aka Internal Friction.
;-)
|
|
|
|
|
|
|
