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post #16 of 28 (permalink) Old Feb 1st, 2011, 06:33 PM
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My Coopper S Convertible 2006 was particularly thirsty for the first 5000 miles and after that...it calmed down. So I'm "prepared" for a thirsty car for a bit!

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post #17 of 28 (permalink) Old Feb 2nd, 2011, 10:47 AM
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I wonder if people complaining about poor mpg are using cruise control?

I use mine in urban areas to help keep within (a reasonable margin of!) speed limits but NOT on open trunk roads or motorways as economy improves if you allow the car to lose speed slightly going uphill and accelerating gently going down. (Depending on traffic conditions, of course.)

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post #18 of 28 (permalink) Old Feb 2nd, 2011, 11:19 AM
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Not a regular cruise control user here

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post #19 of 28 (permalink) Old Feb 2nd, 2011, 10:43 PM
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Quote:
Originally Posted by dlpruk View Post
...as economy improves if you allow the car to lose speed slightly going uphill and accelerating gently going down.
Do you have any evidence for that, because it contradicts most known laws of physics and thermodynamics? Or are you saying this gives the lowest instantaneous mpg reading (which is not necessarily the lowest overall mpg)?
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post #20 of 28 (permalink) Old Feb 3rd, 2011, 02:01 AM
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Quote:
Originally Posted by Angib View Post
Do you have any evidence for that, because it contradicts most known laws of physics and thermodynamics? Or are you saying this gives the lowest instantaneous mpg reading (which is not necessarily the lowest overall mpg)?
I can't prove the physics but it is advice I had from an economy run competitor years ago who cautioned against fighting gravity by increasing power and fuel consumption uphill - as cruise control has to do to maintain a constant road speed - but taking advantage of it downhill to increase road speed with less power/fuel input and thus maintain the same average speed.

It seemed logical at the time and still does to a simple mind like mine!
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post #21 of 28 (permalink) Old Feb 3rd, 2011, 07:41 AM
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As above, when travelling at a constant speed, the biggest impact on fuel economy is the amount of throttle opening used to maintain that speed. If you look ahead and see the road goes uphill, you can elect to allow the car to lose speed (maintaining a constant throttle opening) and if the road goes downhill, you can allow the car to build speed (reducing the throttle opening). With downhills, you can remain off the throttle longer as once you get on the flat you can allow the built speed to drop until you are at your cruising speed again. Cruise control simply maintains the speed and as such uses more fuel.

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post #22 of 28 (permalink) Old Feb 3rd, 2011, 11:24 AM
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The problem with not using the cruise control is that the human foot is much less sensitive to differences in throttle opening than the cruise control is.
For a professional hyper miler then not using the cruise control and wearing very thin driving shoes on long driving runs in quiet traffic will undoubtedly yeild better results but for day to day economy driving the cruise control is going to provide more consistent results particularly if you live in an area that does not have many hills.
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post #23 of 28 (permalink) Old Feb 3rd, 2011, 03:18 PM
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Originally Posted by scillyisles View Post
The problem with not using the cruise control is that the human foot is much less sensitive to differences in throttle opening than the cruise control is.
That's certainly an interesting thought. I wonder whether electronic drive-by-wire throttles have gone some way to reduce this sensitivity?

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post #24 of 28 (permalink) Old Feb 3rd, 2011, 09:08 PM
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Am I the only person who is certain that cruise control makes fuel economy worse, not better?
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post #25 of 28 (permalink) Old Feb 4th, 2011, 09:06 AM Thread Starter
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Am I the only person who is certain that cruise control makes fuel economy worse, not better?
I am sure Mr Clarkson did something in an A8 diesel which proved fuel economy is worse when cruise is used. Though on average speed camera roads it's a godsend :-)

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post #26 of 28 (permalink) Old Feb 6th, 2011, 10:06 AM
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I'm not surprised if people find it more economical to slow down on hills - that gives a lower average speed and a lower speed gives greater economy. Setting the cruise control to that lower average speed would be even more economical.

I suspect the hill advice comes from someone looking at the instantaneous fuel economy readout - it certainly sounds like it. It's very hard for folk to understand that the best mpg reading now may not be the best overall - after all the best possible mpg reading comes from the very worst fuel economy behaviour, using the brakes.

There are two separate factors at work: how much energy the car needs to move and how efficiently the engine can turn fuel into energy. Contrary to what most people think, engine efficency is best at full throttle and peak torque, so pressing the accelerator pedal less is not a guarantee of better fuel economy.

That's why the real fuel economy experts, doing things like the Shell Eco-marathon at several thousand (yes, thousand) mpg use a burn and coast technique where they accelerate hard up to 20mph, switch off their engine and then coast down to 10mph before starting the cycle again. Doing this in normal cars gives fuel economy in the hundreds of mpg - but it's a crap way of getting anywhere.
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post #27 of 28 (permalink) Old Feb 6th, 2011, 10:52 AM
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Quote:
Originally Posted by Angib;
3790710There are two separate factors at work: how much energy the car needs to move
That's why I think easing-off uphill and accelerating down to slightly more than the desired cruising speed gives greater economy for a given average speed than cruise control can achieve by maintaining a set speed.

Someone else can do the maths but if the overall energy required to move a car is the sum of the energy needed to overcome air and rolling resistance plus that required to lift the mass up the hill - remembering that power = the rate of doing work, if I remember my school physics teacher.

Reducing the kinetic energy input (I.E. road speed if the mass remains constant) uphill must therefore reduce fuel consumption at the expense of speed and the latent energy gained by the top of the hill becomes available to increase speed going down again.

I think!

Last edited by dlpruk; Feb 6th, 2011 at 10:55 AM. Reason: Correcting quote
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post #28 of 28 (permalink) Old Feb 10th, 2011, 06:22 PM
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Returning to the MPG topic

I've got a petrol Cooper Countryman.
Done just over 500 miles and I've calculated the mpg for the first time - and I'm pleasantly surprised.
Computer says 38 MPG.
Actual, calculated, mpg - 40 !!

My wife has done most of the driving and is light footed - but she did most of the driving in our Qashqai and we only ever got around 36 mpg in that.

Things might even get better after the running in period too.

So, sub 40 in a Diesel sounds a bit poo to me.
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