I was always taught that the engine is at it's most efficient at the revs where maximum torque occurs - but modern engine design has pushed the max torque revs right up, in order to meet ever tighter emission rules.
Anticipation is the way to drive, avoiding braking unneccessarily by slowing (or gently accelerating) into natural gaps in the traffic.
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RT is correct re max efficiency at max torque. My car handbook states the rpm at which this occurs but as it is a diesel, with a fairly flat torque'curve', it isn't too relevant!
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The best thing you can do to make your driving greener is to buy a second hand car and keep it for as long as you can.
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I've often heard people quote "most efficient at the revs where maximum torque occurs" and then make the assumption that you need to floor it in every gear to get the best ecomony. This is wrong!
At high revs the engine is making the best use of the fuel to produce power, the problem is that it is also using the maximum amount of fuel!
Real world experience proves that cars give the best economny if you change up early.
Another key point to remember is that its the overall driving style that makes a big difference. Read the road ahead!
Someone on here said "the right pedal uses petrol, the middle pedal wastes it" - I thought that was an excellent quote.
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Someone on here said "the right pedal uses petrol, the middle pedal wastes it" - I thought that was an excellent quote.
Very good statement. So basically green driving is a matter of using the right pedal sensitively and the middle pedal as little as possible.
However, very slow aceleration and absurdly low cruising speeds aren't necessarily economical. There does come a speed where aerodynamic drag begins to cost a lot of fuel, but it is quite high with most modern cars. Dawdling down main road hills at 50 or less just means you have to press the pedal harder going up the slope ahead. Being really, really stupid and wimpish does not lead to economical driving, and is bad for other people's equanimity.
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The best thing you can do to make your driving greener is to buy a second hand car and keep it for as long as you can.
Great, I'll nip out and get an old MOT failure to keep on the driveway while I drive around in my 4x4. That'll show them greenies I care about the planet.
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"In theory, I believe the most efficient way is to floor it, but that's not much good for the car." Why on Earth would you think that flooring it is the most efficientway of accelerating? If you remember your Physics from school, the faster you change an object's velocity the more force it takes.
Firstly, I didn't want to talk about economical driving in general, but just about this particular piece of advice from the radio (but all comments are still welcome). 'Acceleration sense' is clearly the real way to make a good saving, but you can't really explain that in a radio advert.
You are right about the physics, hard acceleration will use more fuel, but accelerating hard will mean accelerating for a shorter period of time. Would it be better to accelerate hard for 10 seconds, or gently for 15-20 seconds? I seem to remember seeing the most efficient vehicle a while ago, the driver (a child) accelerated at full throttle upto a certain speed, switch the engine off, let the car gradually slow up, and then start again at about 20mph.
I would expect cars to be most efficient when it's getting max torque, which can be just about anywhere between 2000 and 6000rpm on most cars. Having said that it may not be the most efficient place to change up a gear.
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You are right about the physics, hard acceleration will use more fuel, but accelerating hard will mean accelerating for a shorter period of time. Would it be better to accelerate hard for 10 seconds, or gently for 15-20 seconds? I seem to remember seeing the most efficient vehicle a while ago, the driver (a child) accelerated at full throttle upto a certain speed, switch the engine off, let the car gradually slow up, and then start again at about 20mph.
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I believe that was the way they used to drive on those Mobil economy runs, in cars specially tuned to do it and undriveable in a normal way. Must have been extremely nasty and boring driving like that.
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"Change up gears earlier" is a statement that may mean different things to different people.
Most cars spend such a small amount of time accelerating that I wonder if it makes much difference what method you choose. Accelerating briskly with 1/2 - 3/4 throttle between 2500 - 4000 rpm (in a petrol) seems to make most sense to me.
Minimising the use of the brakes seems more advantageous, doing so will also minimise brake pad wear and tyre wear. You could also avoid enthusiastic cornering speeds to reduce tyre wear.
Was this child driving one of those very aerodynamic vehicles powered by a lawnmower type engine?
Purhaps not relevant to cars but when i go on a First Great Western diesel train, once up to speed the driver backs right off the accelerator and the engines revert to idle for a minute or so before accelerating again, but being a train (weighing approx 100 Tonnes per carriage) it loses very little speed during these coasting periods.
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"Change up gears earlier" is a statement that may mean different things to different people. Most cars spend such a small amount of time accelerating that I wonder if it makes much difference what method you choose.
True, but they also use a lot more fuel when accelerating. In built up areas you can also be doing a lot of accelerating.
Accelerating briskly with 1/2 - 3/4 throttle between 2500 - 4000 rpm (in a petrol) seems to make most sense to me.
That's probably reasonable.
Minimising the use of the brakes seems more advantageous, doing so will also minimise brake pad wear and tyre wear. You could also avoid enthusiastic cornering speeds to reduce tyre wear.
>>Was this child driving one of those very aerodynamic vehicles powered by a lawnmower type engine?
Don't know, I can barely remember it.
Purhaps not relevant to cars but when i go on a First Great Western diesel train, once up to speed the driver backs right off the accelerator and the engines revert to idle for a minute or so before accelerating again, but being a train (weighing approx 100 Tonnes per carriage) it loses very little speed during these coasting periods.
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Just read this same question in the Sunday Times driving supplement today. It is recommended that you engage 1st til 2000rpm, then 2nd til 2000rpm, then 3rd til 2000rpm etc.
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Youcould also avoid enthusiastic cornering speeds to reduce tyre wear.
I tend to keep corner speeds quite high (but maintaining safety) as it means I have to accelerate less, therefore keeping fuel economy up.
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Advising drivers to change up at low revs is potenially dangerous as in higher gears at low speeds control over the car is reduced. The Aygo will do 30 in 5th, excellent for fuel economy, but nil response if you put your foot down and no felixibility - 3rd is far more sensible.
HM Govt may as well say out cars into neutral when going downhill to reduce the revs to tickover and save even more fuel.
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HM Govt may as well say out cars into neutral when going downhill to reduce the revs to tickover and save even more fuel.
Ah, but theat will INCREASE fuel consumption in modern cars. When you coast in neutral, the engine uses fuel to stop itself from stalling. However, go downhill but stay in gear and the engine will use zero fuel provided you don't touch the accelerator.
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Changing up early could be dangerous?
How on earth can this be dangerous? How on earth can control be reduced? You still have full control of the vehicle regardless of what gear it is in. And don't give me that 'you can accellerate out trouble' line - that's complete rubbish. Accellerating out of trouble normally means you've pulled out on someone at a junction becuase you didn't pay attention in the first place.
Coasting with the car in neutral makes a huge difference, but it takes practice to do it safely. You can also use the 'speed up then coast,then speed up again' method, but again it takes practice and you really need to be aware of other drivers (and be considerate to them).
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Changing up early could be dangerous? How on earth can this be dangerous? How on earth can control be reduced? You still have full control of the vehicle regardless of what gear it is in. And don't give me that 'you can accellerate out trouble' line - that's complete rubbish. Accellerating out of trouble normally means you've pulled out on someone at a junction becuase you didn't pay attention in the first place.
You can accelerate out of trouble (if needed), but you can also slow a lot easier and quicker without changing gear. In built up areas where you may need to change between 20-25 and 30mph it can make a lot of sense to stay in 3rd, not use the brakes and not need to change gear. Also in 4th at 30mph the car can be a lot harder to control, a slight change in revs won't effect the engine noise much but the speed will increase a lot more than it would in 3rd. If the speed drops much below 30mph in 4th then you may need quite a bit of gas to bring the car back to 30mph.
Using high gears goes against almost everything taught in advanced driving. I don't use a high gears, I always use the most appropriate gear for the situation. In my opinion the safety gained from a lower gear for outweighs the benefits of a higher gear, even if it's just to keep the maniac behind happy.
Coasting with the car in neutral makes a huge difference, but it takes practice to do it safely. You can also use the 'speed up then coast,then speed up again' method, but again it takes practice and you really need to be aware of other drivers (and be considerate to them).
I really doubt that this is a good idea. I would be quite supprised if there are any benefits, but in addition to this there will be wear on the clutch etc., and the saftey issues.
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"You still have full control of the vehicle regardless of what gear it is in."
I suggest a descent of Porlock Hill using top gear then come back and say you had full control of the vehicle.
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I assume Porlock Hill is very steep, the brakes on my car work fine but of course driving at 20mph down a steep hill in 5th would be crazy. You are completely missing the point, you change into the highest gear possible for the conditions, meaning you don't just stupidly drive around in 5th all the time. You need to apply common sense and not take the instructions literally - and we wonder why we are becoming the nanny state....
Coasting - why does it cause more wear on the clutch? I think you are talking complete rubbish, but feel free to prove me wrong.
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Coasting - why does it cause more wear on the clutch?
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Because you are either making two additional engagements of the clutch (if you coast with the gears in neutral) or you are holding the clutch disengaged for a long time (if you coast with the clutch depressed).
Either way, when you re-engage the drive chain there will be mismatch of plate/flywheel speeds, unless you rev up the engine to match the speed. I suspect you would waste more fuel than you saved.
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I coast in neutral, dis-engaging the clutch will cause such a small amount of wear that its not worth mentioning (release bearings seem to be of good quality these days so no wear there either). So the only possible wear is when you put it back into gear, to my mind this is minimal and is no different to a normal gear change. My car now has 110K on the clock and is on the original clutch and no signs of any problems what-so-ever.
As for it using more fuel - I've managed my best ever mpg by use of coasting, it makes a bigger difference than you may think.
One day I might fit a video camera to my car a film a selection of my journeys home from work with different driving styles and conditions. You would need to see both the road ahead and the MPG gauge on the dash to get the idea. I think too many people on here competely fail to see how all this works.
Heavy on the gas, heavy traffic and failing to read the road = poor MPG (for me that means 25-30mpg)
Light on gas, change early, coasting, read road ahead, light traffic = good mpg (45-50mpg)
Of course there are loads of levels in between the two extreme examples above. E.g coasting or driving at 50mph on NSL dual carriageway in rush hour just isn't going to work (assuming you get to go that fast anyway).
BTW - I like to get where I'm going quickly and I'm not one these people who drive at 40mph on a dual carriageway to save a few pence on fuel :)
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So the only possible wear is when you put it back into gear, to my mind this is minimal and is no different to a normal gear change.
When you are coasting in neutral presumably the engine is idling, at say 800 rpm? After you have re-engaged gears it might need to be doing perhaps 3000 rpm? Do you simply slam it into gear, and let the synchromesh take the strain, or do you speed up the gear train first by giving a blip on the throttle?
I can't believe any of this is good for the gearbox, clutch, or saves fuel. Your car has done a very small mileage which frankly proves nothing. Tell us when you have clocked up 370,000 miles.
I do agree however that anticipation is the clue to economical driving.
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It has already been pointed out that modern injected cars use fuel when idling but don't use any fuel at all on the overrun: if you leave the car in gear with foot off when going down a hill or slowing, no fuel should be being used until the engine slows to idle speed.
So coasting with the engine idling and out of gear may be using more fuel than staying in gear.
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It would seem that I disagree with everyone on this board at the moment :)
Coasting in my experience uses less fuel. This is based on my real world experience, not theory. Yes, fuel injected cars (which mine is) do shut off the fuel on the overrun. But, you are missing a point - with the car out of gear and coasting there is no engine braking effect on the car and you can build up momentum, hence you can then travel for further before you need to hit the gas again.
I have tried many methods over the years (to ease the boredom of my commute every day) and without a doubt the methods I use are the best. I have been driving the same journey 5 days a week for 8 years and the last 5 have been in my current car.
I still maintain that my methods give the best MPG if used correctly and with a good helping of common sense and consideration for other road users. In fact they are not even my techniques, it's all common knowledge stuff. I understand that the guy who holds the world record for most MPG uses these methods, I'm sure someone here will know more about it.
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The Sunday Times "suggested" change point is daft - except for my diesel. However, even that I think is better taken to 2500. However one of my petrol engines would be seriously labouring if I did it - and it's only 8 valve as well. 3000 in first and second, and 2500 in third and fourth please.
Anticpation, and avoiding the need for braking, so you don't have to accelerate again are the most practical skills for most of us.
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My own methods are as follows:
* Light right foot.
* Anticipating the braking as much as possible.
* Using Premium Petrol [ BP Ultimate / Shell V Power ].
* Open the window a bit instead of using the air blowers.
* Not using the radio at all [ unless SWMBO is in the car! ]
* Trying to choose quiet times to drive so that speed can be kept constant.
* Similarly, forget slipstreaming - getting clean air into the engine is not only more efficient but also safer [ greater distance to vehicle in front! ].
I have a 10 year old Suzuki Swift automatic and I typically get 40 - 45mpg out of it, but have pushed it past 52mpg on a long trip in ideal conditions with empty roads recently. the previous owner [ my father in law actually ] says he could normally only get 30 - 35mpg from it - although that is not a direct comparism as he used normal petrol.
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I guess that Moonshine's coasting technique is successful in the context of fuel economy but coasting is anathema to me. None of the 'driving manuals' or similar recommends it. I suggest that a coasting car would not be as stable as a 'driven' one during a sharpish directional change, such as one occasioned by some sort of hazard that arises unexpectedly. OK, low risk aybe but...
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Absolutley! reading the road ahead and maximum concentration is needed when coasting. I'm not saying it's safe, I'm saying it saves fuel.
it's very embrassing when you forget that you are in nuetral and rev up to the redline and go nowhere. It's a mistake I only made once!
I could argue that by paying so much attention to reading the road (ahead and behind is important, oh and don't forget the sides!) that you are in fact a much safer driver.
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>>* Open the window a bit instead of using the air blowers.
Won't that use more fuel? The drag will increase, especially at high speeds. In stop-start traffic this is probably a good idea, but over 20-30mph I would have thought the drag would use a lot more fuel than the blower. With the blower on either 1 or 2 (3 and 4 is stupidly powerful) the engine barely notices the difference. Open the window at 60-70mph, and I will quite easily notice a difference with the engine.
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I once had a classic Saab 96 with the freewheeling lever. It was OK hurtling along silently with the engine ticking over, but as soon as I needed to re-engage gears (eg if the road started going up) I had to rev the engine back up to gear-train speed. Surely a steady throttle opening wastes less fuel than speeding the engine up and down ?
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P3t3er's point about lower gears is a good one, especially in 30mph areas. I suspect the complaint you often hear, that 30 is a difficult speed to stick to, tends to come from people who try to drive at 30 in 4th. I used to do this, until a day on a defensive driving course with an ex-police instructor convinced me otherwise.
In my present (diesel) car, driving in third feels absolutely right - 30 is well into the engine's torque band, so I can back off the throttle when necessary, get the benefit of extra engine braking, so use the brakes less, and have torque in reserve to get smoothly back to 30 without labouring or changing down; once the speed rises above 30, though, the engine gets eager and starts making its 'rev me' sound, which is a useful warning that I need to back off a little.
As for efficiency, HJ in his DT column often recommends 2000 rpm as the optimum speed for a modern diesel, and that seems to work with mine. Last weekend I drove 200 twisty, motorway-free miles home from Pembrokeshire and used 5th only when I was sustaining a speed of 50 or more. (50 is about 2500 rpm.) That gave me better-than-average mpg and what felt like smoother progress, so lower gears generally get my vote - although I do occasionally wish I had a higher top on the motorway to bring my cruising speed down towards that 2000 rpm mark again.
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Won't that use more fuel? The drag will increase, especially at high speeds. In stop-start traffic this is probably a good idea, but over 20-30mph I would have thought the drag would use a lot more fuel than the blower.
Not really - I only open the window about one inch maximum, just enough to keep a supply of fresh air flowing inside but without causing a hurrican around the cabin.
I was doing this when I got that 52+ mpg out of it a couple of weeks ago.
It may be different for a car with a bigger engine; I perhaps should have reminded people in my posting that I have a 993cc 3 cylinder version?
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