Utter garbage of course.
Why?
Only a person without the smallest degree of mechanical sense could believe that of any car made after 1960 (...)
Oh dear ;)
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Why?
Others can give you a better explanation than me. But I suggest you try not exceeding 40 on your next journey of over ten or fifteen miles, and then try the same journey not exceeding 60, driving in both cases in a light-footed 'economical' manner without going to economy run extremes (e.g. changing into top at 15 mph and then driving at full throttle to 35 before turning the engine off and coasting back down to 15 mph, or I think it was something like that).
Of course it would be very easy to make cars which were more efficient at a mimse. It's just that most of them aren't like that at the moment, thank goodness. I would hate to think crawling carphounds were getting any encouragement from the industry as well as the ghastly politicos and safety wonks.
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I'm an electronics engineer (rather than mechanical) but I've done enough mechanical engineering and physics to know that lower speeds SHOULD be more efficient.
Aerodynamic drag (which is the main form of drag on a car at cruising speeds) scales as the CUBE of the velocity. So a car going 50mph might need 10kW to overcome aerodynamic drag, but at 100mph it would need 80kW to overcome the drag - the power requirement rises massively.
There are also other drag effects (e.g. fluids in the engine and transmission, bearings), and I suspect that they would scale in a similar way. I can't think of any drag factor which would have an inverse relationship with speed....
Obviously torque and power curves come into play (gearing) but the effect would be relatively small.
I know there are some people who find their car more economical at 90mph than at 50mph, but these are the same cars that can stop four times fast than anyone elses etc etc. (As Mr Scott said, "Ye cannae change the laws of physics").
Most of the fuel you use goes into acceleration. Once up to steady speed the power requirement is much lower. A constant low speed (in appropriate gear) should give best economy.
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I think we have to balance fuel efficient with time efficient and capacity efficient. Time is money, and productive people can offset the extra fuel used. Also, if you reduce the speed on a main arterial road which is at near capacity (as most are) the feeder roads get jammed (as we have seen since the NSL was unofficially all but abolished)
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Aerodynamic drag only starts to become significant over about 55mph with an ordinary, non-brick-shaped vehicle. The high gearing of modern Euroboxes makes it difficult to drive them in top gear without labouring the engine at these very low speeds, especially up any sort of incline. Driving them in lower gears prevents labouring and enables a light foot to be used, but they will still use more petrol.
On the road, I often see these mimsing drivers applying their brakes downhill to avoid exceeding some pre-set speed, and then having to press the loud pedal quite hard to labour up the hill opposite (and often forcing others to do the same). This isn't just aesthetically offensive and obstructive to other traffic, it is heavier than these people think on fuel.
After driving for nearly 50 years, I have concluded that the most economical driving is light-footed, elegant and a bit quicker than most people usually go except on motorways or fast dual carriageways. French A roads often return astonishing mpg figures when you aren't in a hurry.
The elegance is very important because you need thought and anticipation to maintain it. Alas, there are many who wouldn't recognise elegance if it surged up wearing a Vivienne Westwood creation and bit them in the bum.
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Aerodynamic drag only starts to become significant over about 55mph with an ordinary non-brick-shaped vehicle. The high gearing of modern Euroboxes makes it difficult to drive them in top gear without labouring the engine at these very low speeds especially up any sort of incline. Driving them in lower gears prevents labouring and enables a light foot to be used but they will still use more petrol.
Doing a few calcs (and asuming CD~0.33) suggests that aerodynamic drag is about 50% of total drag at 50mph. It will still be significant at 40mph. Think about driving in the car when there is a 40mph crosswind - you really feel it. These are not insignificant effects.
Most cars should be able to pull 40mph in top gear comfortably.
Not saying that people SHOULD drive at these speeds, but they are likely to get good MPG.
Basically for good MPG on the open road you need good aerodynamics. For good MPG in town you need to reduce mass (weight).
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I have concluded that the most economical driving is light-footed elegant and a bit >> quicker than most people usually go except on motorways or fast dual carriageways. French A roads often return astonishing mpg figures when you aren't >> in a hurry.
The elegance is very important because you need thought and anticipation to maintain it. Alas there are many who wouldn't recognise elegance if it surged up wearing a Vivienne Westwood creation and bit them in the bum.
That's the finest thing I've heard today.
(it is only 8am though)
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It does trouble me though that Lud (of whom we are not worthy) can associate V Westwood with elegance. I'd have thought, say, the recently departed YSL would have been more within his frame of cultural reference...
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Like biting someone in the buttock, wearing Vivienne Westwood creations is noticeable rather than elegant. I thought people would recognise that.
Speed up Eddie... :o}
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But no offence Eddie... :o}
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The danger with these simplistic & dogmatic one-size-fits-all nostrums, as in the WhatCar 'revelations' about driving more slowly in higher gear, is that is exactly how the vast majority of the mechanically illiterate will behave. I could imagine many will interpret 'get into a high gear ASAP' as 'ignore commonsense & labour the engine' , with the 'max. mpg @ 40' as 'crawl up any incline & lose speed'.
As Lud points out - light-footed anticipatory driving with an engine spinning between its bhp/torque peaks often yields best (least) consumption. The WhatCar piece can't be faulted on the basic physics, but it made far too little of the 'art' of driving. Lud rightly uses the term 'elegant', not in reference to graceful hand/foot/eye movements & co-ordination (although that could be a factor..!) but in the application of efficient movement through a crowded car-scape.
Mind you, for me, WhatCar has a very mundane & unromantic view of 'motoring' - almost like it should be done through gritted teeth & with the sour expression of someone taking a foul medicine.
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a very mundane & unromantic view of 'motoring' - almost like it should be done through gritted teeth & with the sour expression of someone taking a foul medicine.
A fine description of the way a lot of people drive woodbines. I do wish there was better public transport so the poor darlings didn't have to do it and get in one's way.
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Light on the throttle, antisipation of delays aheads (lights, trafic crossings etc) and the minimal use of the brakes (lose your inertia and you're back on the throttle again).
A wide open throttle at 30MPH in sixth would use more fuel than a partially opened throttle in fifth because the engine would be outside its power band
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Light on the throttle, antisipation of delays aheads"
"It still astonishes me how many people drive at speed (and even accelerate) towards red traffic lights and then having to break. Instead of lifting off the accelerator and slowing which might even mean you do not need to completely stop. It's the same when approaching stop/give-way lines too with you thinking they're not going to be able to stop.
Edited by rtj70 on 11/06/2008 at 16:38
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Others can give you a better explanation than me.
I doubt it, unless it's a similarly fallacious one.
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Enjoy the odd fallacy do you FT? So do I, so do I.
:o}
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The most efficient speed seems to be below 40MPH for many, if not most, cars.
We need something like "Driving right violation" similar to "Human right violation".
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About 55 mph seems to be most economical for my landrover (don't all laugh). It's just below the torque peak of the engine, before the aerodynamics of a garden shed come much into play.
40 - would be a gear lower.
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The Times had an article on it as well (perhaps the same source?) and hidden among the headline 20mph/100mpg were the two key points:
- Smooth progress (ie constant speed if possible) gave the best results
and
- Use the correct gear for the speed you are travelling at, though it did generalise a bit there.
Cor, Wow!
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Even if we agree 40mph is the most efficient speed (which I doubt!) then what does it actually mean? Are we all going to sit on the motorway at 40 to complete our 300 mile drive?
Its all relative to the real environment you're in. If I have a clear motorway in front of me, I might cruise at 80. Usually I end up at 50-60 anyway.
Imagine doing 40 mph with an empty motorway stretching out in front of you....
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Even if we agree 40mph is the most efficient speed (which I doubt!) then what does it actually mean? Are we all going to sit on the motorway at 40 to complete our 300 mile drive?
Please, please, please, don't give this government any ideas about future motorway speed limits!!!
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I would think the most economical condition is a combination of the highest gear practicable together with the smallest possible throttle opening.
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>>smallest possible throttle opening.
That's certainly not correct for a petrol engine, where one of the causes of inefficiency is the pumping work done against inlet manifold vacuum.
It's quite true that drag power goes up with speed, and so, less fuel is wasted to drag at low speeds.
To clarify, aerodynamic drag force is proportional to speed squared, which makes the power required to overcome aerodynamic drag proportional to speed cubed.
Yes, there is a sweet spot in an engine's performance map - typically minimum consumption occurs at about 70% load, and at speeds close to the engine's peak torque point. To exploit this fully, a CVT gearbox is required. The CVT should allow the engine to rev to peak power to obtain the best acceleration, and should then seek the minimum fuel consumption range during cruise.
With a manual gearbox, the ability to take advatage of the optimal engine operating point is much limited, and keeping cruising engine speeds at or below the speed for peak torque is probably not a bad start.
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