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    • CommentAuthortony
    • CommentTimeJun 23rd 2008
     
    There have been several references recently to fuel consumption and speed -- driving a 56mph is as we all know more fuel efficient than 70

    What speed should we drive at?

    Should there be a lower speed limit on motorways?

    Could we change the world by all driving a bit slower?
    • CommentAuthorStuartB
    • CommentTimeJun 23rd 2008
     
    Some people would love to be able to drive at 56mph commuting to work. Most of the time it's 10-20 stop start!
    • CommentAuthorchuckey
    • CommentTimeJun 23rd 2008
     
    Yesterday evening, while following some cars doing 35 mph on a twisty country road I came to the conclusion that there should be a minimum speed for road users. If they are not in a hurry, they can stop and have a look at the countryside and then drive at a reasonable speed, say 50 mph. If they are not confident to drive at that speed then they should stop driving completely.
    Frank
  1.  
    Although I do drive at 70 on motorways, I would be more than happy to drive at 56mph if I could get there without delays. I'm not sure how prevalent that view is though - I can't imagine white van man, boy racers and company car drivers who live in their cars agreeing.

    On a slightly separate subject, I would introduce a noise element to the VED tax - no noise = no premium; boy racer/4x4 noises would add premiums. So we could have a double benefit... improved fuel economy at 56mph and improve environmental stuff (less noise and other pollution).
    • CommentAuthorGBP-Keith
    • CommentTimeJun 23rd 2008
     
    Ban everyone from driving until they are 50! Have you guessed that I have a birthday coming up soon?

    Joking aside. driving slowly is an art that can only be mastered with age.
    • CommentAuthorGBP-Keith
    • CommentTimeJun 23rd 2008
     
    Will I need to call in the grim reaper soon?

    Some threads are being introduced off subject.

    Perhaps we should start an offshoot forum on green living? I'm happy to discuss this till the cows come home but on a green building forum people will expect a tight ship or the captain might be keelhauled.
    • CommentAuthorBowman
    • CommentTimeJun 23rd 2008
     
    35mph on a twisty country road? Sounds okay to me. Five accidents this year on country lanes, the first four I was stationary before being hit, the last wrote my car off. Oh and been hit twice as a pedestrian too.

    My wife and I swapped cars for a bit in a mpg competition, I got 39 to her 33 in a bog standard Corrola.
  2.  
    I've recently discovered a sixth gear on my Audi... its called neutral and I use it for all down hill sections and for coasting up to junctions. I'm not sure that this is good driving practice but it's pushing the mpg up.
    •  
      CommentAuthornigel
    • CommentTimeJun 23rd 2008
     
    Most cars cut off the fuel supply when slowing down so I suspect your idea does little to reduce fuel consumption and is defintely not good driving practice.
  3.  
    Posted By: Chris WardleI've recently discovered a sixth gear on my Audi... its called neutral and I use it for all down hill sections and for coasting up to junctions. I'm not sure that this is good driving practice but it's pushing the mpg up.


    It's not just not good practice, it's dangerous and illegal and doesn't save any fuel. According to the highway code, you should be in control of your vehicle at all times - coasting in neutral is not being in control. It's especially dangerous going downhill as you may find the lack of engine braking overheats your brakes - I was in a car that this happened to (as a passenger) and I can tell you that was the last time that idiot driver ever coasted down a hill.

    As for saving fuel, you'll use more fuel when coasting, not less. Since fuel injection and engine computers became standard features, all fuel injection is shut off if the car is decelerating whilst in gear - i.e. zero fuel consumption. If you coast, a small amount of fuel will be burned to keep the engine turning at idle speed.

    Seriously, do not ever consider coasting - it's not economical and it is illegal because it is dangerous.

    Paul in Montreal

    p.s. and as for driving speed, I think there should be no limit except where dictated by safety considerations. If the speed limits were increased, the capacity of the roads would increase and this would probably lower overall fuel consumption, not raise it. If I do a cost benefit analysis of the differential cost of extra fuel used due to driving faster, I find it is always much less than the value of my time at the standard rate I bill clients. The cost I pay in taxes is directly related to my consumption (and hence also to CO2 emissions) so there's no need for any extra taxes - we already have de facto carbon taxes on fuel usage. It's up to me how I use that - sometimes I drive slowly and "economically" and sometimes it's more economic for me to drive fast - especially here in the Great White North where the roads are virtually empty - even between major cities
    • CommentAuthorludite
    • CommentTimeJun 23rd 2008
     
    OFF SHOOT FORUM ON GREEN LIVING!!!!!! You mean to say, I won't have to waste my time looking for an alternative green living site to go with this one????? Can I be a Ludite on both of them????? FAN-TAS-TIC idea.

    Green building should stick to the subject, but the problem is: Nothing can be taken in isolation. The more you read this forum, the more ideas you get. Some of them are not related to building, but the people who come on this forum would know the answers. . . . . . if only you could ask the questions. . . . . .
  4.  
    Ah... better pack that in then:cry:
  5.  
    It all depends on how much you value your time. I know is not as simple as this sum, but if I do 140 miles at 70mph, it takes me 2 hours. If I do it at 56mph it takes 2 hours 30 minutes. How much fuel do I save - suppose its 36.66mpg vs 50 mpg, (and its not actually that bad) I am looking at 4 gallons vs 2.8. Thats 1.2gallons = 5.4 litres = £6.48.

    I don't know about you, but I value my time at a bit more than £13 per hour. And if there are more than 1 person in the car, the sums change again.

    Tim
    •  
      CommentAuthornigel
    • CommentTimeJun 23rd 2008 edited
     
    Posted By: Paul in MontrealIf the speed limits were increased, the capacity of the roads would increase and this would probably lower overall fuel consumption, not raise it.


    I think you will find that the capacity of a road actually reduces the faster the speed limit is. This is because the faster you drive the greater the stopping distance and the greater the gap needed from the car in front.

    The most important aspects of eco driving are driving smoothly, change up early and plan ahead to avoid unnecessary braking.
    I think that the current speed limits, if adhered to, are about right.
    • CommentAuthorSimonH
    • CommentTimeJun 23rd 2008
     
    I think a green living forum would be a great idea. I do post on some other forums but people on this forum seem to have studied the subject matter (general green issues) in greater detail. Maybe because building is technical in nature, GBF attracts a more technical kind of person - who's not so interested in which type of cabbage plant is better, or whether hemp is better than straw for making socks. People are interested in the bigger issues around energy conservation whether that be for cost or environmental reasons. It could mop up the subject on politics, transport and food that default to the general forum at present.
    • CommentAuthorbiffvernon
    • CommentTimeJun 23rd 2008 edited
     
    Posted By: (GBP) KeithWill I need to call in the grim reaper soon?
    No, 50 is still quite young. Encourage folk to use descriptive thread titles then if you don't care about topics such as "How fast should we drive?" you can esily skip it without wasting your time.
    •  
      CommentAuthorrogerwhit
    • CommentTimeJun 23rd 2008
     
    Posted By: nigelThe most important aspects of eco driving are driving smoothly, change up early and plan ahead to avoid unnecessary braking.


    That's realistic. Going slowly would be equally important in an ideal world, ie if everybody did it, but in today's reality it makes those driving up your arse so impatient that they use more inefficient methods (braking alternated with clog-down surges) that the net result is zero.
    • CommentAuthorTheDoctor
    • CommentTimeJun 24th 2008
     
    Surprisingly, it was the green anti-christ Jeremy Clarkson who alerted me to the fact that you get zero fuel consumption when travelling in gear and decelerating.

    He was attempting (and succeeded) in driving from London to Edinburgh and back on a single tank of fuel in an Audi something or other a while ago.

    I always used to coast, but have noticed a marked improvement by just cruising down hills in 6th, and up to red lights.



    so there you go
  6.  
    Having looked into it, its seems that is right (about using zero fuel when decelerating). The only circumstances where coasting would work (in reducing fuel consumption) is when you are going down a gentle slope where the engine braking would reduce your speed such that you have to stay on the throttle a bit to keep your momentum up. In that case, coasting in neutral, while using a bit of fuel to keep the engine ticking over would allow you to get down the hill without touching the accelerator and overall be more economical.

    This is what the UK Highway Code says on the matter:-

    "122
    Coasting. This term describes a vehicle travelling in neutral or with the clutch pressed down. It can reduce driver control because

    engine braking is eliminated
    vehicle speed downhill will increase quickly
    increased use of the footbrake can reduce its effectiveness
    steering response will be affected, particularly on bends and corners
    it may be more difficult to select the appropriate gear when needed"
    • CommentAuthorLizM
    • CommentTimeJun 24th 2008
     
    What's this about reducing the speed from 70 to 56? Have you ever driven on a motorway at bang on 70? Almost everyone passes you, except for lorries, caravans and old people who drive at 60 in the middle lane! Getting everyone to drive at 70 instead of 80 or 90 would be a good start!
    I kind of think it a public duty to stick to 70 as it can reduce the person behind you's speed, thus saving them money too.
    •  
      CommentAuthorOlly
    • CommentTimeJun 24th 2008 edited
     
    Posted By: nigel
    Posted By: Paul in MontrealIf the speed limits were increased, the capacity of the roads would increase and this would probably lower overall fuel consumption, not raise it.


    I think you will find that the capacity of a road actually reduces the faster the speed limit is. This is because the faster you drive the greater the stopping distance and the greater the gap needed from the car in front.

    Do you have any evidence to support this? ...surely German Autobahns would be in a continual state of gridlock if this were the case? :tongue:
  7.  
    The 70 max campaign has interesting thoughts on this: -
    http://www.70max.com/

    Mark
  8.  
    A small contribution........I just had my car converted to gas, cash savings behond expectations. I buy gas for 0.48-0.59/ltr and get better consumption.

    Recommend it to anyone
  9.  
    I was thinking along those lines myself, buildexpress. Can you tell us more, e.g. how much is costs, where the tank goes etc?

    Current car (Audi A4, 1.8 petrol) has done 150k miles but seems to be going well enough. Not sure whether a gas conversion would pay if the engine will perhaps only stand another 30k or so and I'm loathed to part with what has been a reliable motor but which, due to the mileage, has negligible resale value.
  10.  
    I'm with the keeping the limits the same, but enforce them; in particular the low limits (like 20 and 30) - protecting children, and the high speeds (70) reducing fuel and pollution use.

    Speaking for myself coming from a city where I obsessively kept to the limits, due to speed cameras, I do find it hard now living in the countryside and keeping to a 30 limit at 7am on a bright sunny day. I do try though.
  11.  
    to Chris Wardle request.....
    I decided to convert when the tax on larger cars was to be increase. Lets face it, I drive a Jag X type 2.5 Petrol, who would want that with the petrol and tax so high, and again petrol will never come down in price, now it is this high.

    The government are making so much money in tax "as they would say if it was a private company - wind fall tax".
    So i decided to bite the bullet, the cost was £1500.00 and I accepted that this has gone, but a recover the cost each time I fill up or the cost of fuel increases.Its a great feeling, filling up for £30.00.

    The Jag has now a resale value, it cost me less to run than a mini? but I am going to keep it, unless I win the Lottery
    • CommentAuthorgreenman
    • CommentTimeJun 24th 2008
     
    Chris

    I have a Saab 95 converted to run on lpg. It had 52k mies when I bought it - 107k miles now. The conversion cost £2k about two years ago and (because of my horrendous annual mileage) paid for itself in ten months. Unlike buildexpress, I think I get worse fuel consumption (I think they estimate anything from 20% to 30% normally) than I would have done on petrol - this is nothing to do with the conversion, just that lpg has a lower calorific value than petrol. With the petrol/lpg price differential being so high though, even with worse fuel consumption you still save money.

    Other benefits are; reduced car tak (abou £20 less per year), slightly lower CO2, but significantly lower other emissions, and longer engine life expectancy (due to cleaner burn), though if you only convert at 150k miles, I'm not sure you'd see that benefit!
    • CommentAuthorCWatters
    • CommentTimeJun 24th 2008
     
    Posted By: TheDoctorSurprisingly, it was the green anti-christ Jeremy Clarkson who alerted me to the fact that you get zero fuel consumption when travelling in gear and decelerating.


    I believe that's at least partly correct. The injectors on a modern car can indeed shut off completly when coasting ingear but they can't when coasting in neutral. However I wonder if the drag of the engine allows you to coast as far in gear as it would in neutral? Perhaps the drag of the engine slows you down more so you have to accelerate harder to regain your desired speed.

    Generally driving at a constant speed works best because drag is proportional to the square of the speed and power required is proportional to the cube. Say you want to average 60mph and you drive half the time at 50 and the other half at 70.. the bit you save by driving at 50 is less than extra you burn by driving at 70. So a constant 60 is better.
  12.  
    Posted By: nigelI think you will find that the capacity of a road actually reduces the faster the speed limit is. This is because the faster you drive the greater the stopping distance and the greater the gap needed from the car in front.


    It depends how the speed limit is set. If it is set using engineering criteria, then the capacity can be maximized. Often limits are set arbitrarily low (for revenue collection or other non-engineering reasons) and congestion is the result. People driving too slowly (because they are unskilled or deluded into thinking that if the limit says 70 it is their duty to prevent people from going faster) cause congestion. The safest speed to travel at (and we're talking of motorways here, not urban roads) is the speed of the 85th percentile - irrespective of what the speed limit says. Most motorways in the UK are designed for safe travel at over 95mph (in clear conditions of course) - this speed being determined by the radius of the curves and other sightlines. Stopping distances don't come into it unless there's congestion. If they did, the autobahns would be bloodbaths all the time and they're clearly not. I was with a colleague near Munich in 2005 driving at 255 kph and felt quite safe at that speed (I wasn't driving) - the thing over there is that when there are posted limits, people obey them. Anyone who has it in their mind to block others exceeding the limit has no right, in my humble opinion, to be driving. You have no idea why the person behind you may be going faster that you are comfortable driving and have no place in endangering others by causing congestion by your selfishness.

    Paul in Montreal
    •  
      CommentAuthornigel
    • CommentTimeJun 25th 2008
     
    Paul

    I am having difficulty understanding what you are saying. On the one hand you are saying that the faster you go the greater the road capacity, but only if there is no congestion.
    If there is no congestion no one cares what the road capacity is anyway.

    The M25 western section uses speed management to maximise the capacity of the road and improve flow. Prior to this the road was almost gridlock with stop start traffic trying to go as fast as possible. Speed management in these conditions is good for fuel economy and good for easing congestion.

    I dont think I have ever experienced congestion because the speed limit is set too low.

    The average traffic speed in London according to Citroen is just 7 mph and we blame the speed limit for it.
   
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