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Cruise control and fuel economy  
Cruise control and fuel economy - DP
Just to help settle a pub argument from the weekend.

Two identical cars driving at the same speed on the same road in the same conditions.

One is using cruise control to maintain speed.
One is being driven conventionally with the driver maintaining the speed.

Will there be any difference in fuel consumption between the two cars? If so, why?

A mate seems to think that cruise increases fuel consumption. I can't see how or why this should be the case.

Answers gratefully received.

Cheers
DP


Tags: driving fuel economy speeding

Cruise control and fuel economy - oldnotbold
In fact the cruise control should be better, unless the "manual" driver is very good at maintaining a truly constant speed. While it won't affect fuel economy too much, speeding up to 74 and then dropping back to 67 and then going back up to 72 will consume more fuel than staying at 70 on cruise.
Cruise control and fuel economy - ForumNeedsModerating
Perhaps the only reason it might (..and I've tried an 'identical' journey with it on/off) is that humans can see & anticipate a bit better - and so operate the throttle more delicately (with view to efficiency that is).

Often CC will give a bit too much ooomph going up a hill or rise, whereas a 'human' may just let the car roll up slightly in anticipation of the decline. Numerous other examples as well of similar ilk.

Edited by woodbines on 19/03/2008 at 13:53

Cruise control and fuel economy - Garethj
I've found that cruise control makes economy a touch worse, but as said above, perhaps that's because I ease off a bit on the top of the hill. Not much in it though, and cruise control is fantastic for going through road works at 40mph
Cruise control and fuel economy - perleman
When driving, as you go over bumps, your foot on the pedal moves up and down a bit, constantly using extra bits of fuel

Vs.

The car opens the throttle more than you would due to changes in gradients

Cruise control and fuel economy - MazMan
The only car I've owned with CC so far is Volvo S40 TD and I personally experienced it used to drink more diesel when CC used to be in action and I'm talking about the same speed, same driving conditions etc..
Cruise control and fuel economy - nick62
I find people who don't use or have cruise control tend to slow down on uphill gradients. The cruise control on my Legacy is fantastic, but I have no other car to compare it with
Cruise control and fuel economy - DP
Thanks all. I can feel in the Volvo that it uses more throttle going up inclines than maybe I naturally would, but I think that's because I would probably allow the speed to drop off slightly as nick says above. Technically, the cruise control is driving correctly as it's maintaining a constant speed.

The other thing the Volvo's system does is accelerate quite hard back up to speed when you use the resume function after slowing down while waiting to overtake on the motorway. It accelerates harder than I would if I were driving manually, but then there are also schools of thought that say it is more economical to accelerate quickly to your chosen speed and settle into the cruise as soon as possible, than to accelerate gently but for longer.

This cruise control malarkey is new to me. The Volvo is the first car I've owned to have it, and I am finding that I use it wherever the traffic conditions allow on the motorway. It's also a great tool for travelling between SPECS cameras.

Cheers
DP
Cruise control and fuel economy - David Horn
The Honda's a bit like that, and on a moderate gradient at slow speeds I can feel the accelerator go down to the floor. I will typically knock out the cruise control before a big hill though, or use the manual mode on the gearbox to lock it to 4th gear to avoid the abrupt kickdown.
Cruise control and fuel economy - dxp55
On two Mazda's I have had I only use CC in 40mph or 50mph limits as I found it used more fuel - on the other hand my old Mondeo mk2 2.5 V6 auto was fantastic on CC - don't know if it was anything to do with Ford using windows based system on engine management or not.
Cruise control and fuel economy - Cliff Pope
It's surely wasteful of fuel to try and maintain a constant speed when going up hill?
And silly to rigidly maintain a lower speed on the downhill section when you could be building up momentum for the next rise?

The really interesting bit of kit to have fitted would be Economy Control. It would make no effort to maintain any particular speed, but would always drive at the most economical speed. But probably very frustrating for other motorists.
Cruise control and fuel economy - ForumNeedsModerating
The really interesting bit of kit to have fitted would be Economy Control. It would make no effort to maintain any particular speed, but would always drive at the most economical speed.

I'm pleased to tell you Cliff, that these are already widely available - I've got one it's called a 'my right foot'!

It's surely wasteful of fuel to try and maintain a constant speed when going up hill?

But surely better to try to stay in the powerband & not change down a gear (especially on a diesel)?

And silly to rigidly maintain a lower speed on the downhill section when you could be building up momentum for the next rise?

Most CCs won't 'intervene' on a speed-gathering downhill section - merely take its 'foot' off the gas, although a CVT diesel Audi I had did - to my annoyance, but wonderment.
My c270 will, if provoked by a vertiginous descent, lower a gear - but that happens rarely & only if the speed climbs more than about 10% over set cruise speed.


Edited by woodbines on 19/03/2008 at 15:31

Cruise control and fuel economy - b308
I agree with earlier comments that CC will usually come out on top on a motorway run - certainly that is my experience - I think as a percantage of the whole very few people can keep a car at a constant speed - my family certainly can't, and as any regular user of CC on m'ways will tell you neither can most other drivers - hence use of more fuel - constant slowing down/speeding up without CC!

Edited by b308 on 19/03/2008 at 15:36

Cruise control and fuel economy - craig-pd130
@DP, there was an episode of either 5th gear or Top gear a couple of years ago where they did an economy trial of precisely that point, they found that booting the car quickly up to cruising speed was marginally more economical than easing up to the target speed.
Cruise control and fuel economy - Alby Back
My car does seem to use more fuel on CC. As an aside, CC seems to confuse some other drivers. I often set mine at about 80 on the speedo which is probably in reality somewhere in the mid 70s of mph. I will then find myself overtaking another vehicle on the motorway and it is remarkable how often they will speed up to match my speed thus causing me to either increase my speed to have the opportunity to return to an inside lane or to have to drop back. The other version of this is that you will slide past someone while going maybe 10mph faster on cruise only to be overtaken by them within seconds and for the whole scenario to repeat itself as they slow down again. One guy got really outraged by this on the motorway as he seemed to take offence at my maintaining a steady speed. He would come right up behind me giving it the retina burners, overtake, drop back in, glare at me, be overtaken again by me and then do it again. Ah well, if it brightened his day !

Edited by shoespy on 19/03/2008 at 16:14

Cruise control and fuel economy - Number_Cruncher
The problem is that we can't really compare like with like. The speed control of most drivers is much poorer than even a crude cruise control. If set for a constant speed, cruise control will have some error - it won't keep perfect control, but, a typical driver will be all over the place - typically holding the pedal almost still, and then being caught out if the wind or gradient changes a bit.

The control strategy for the cruise control will more than likely be aimed at providing a smooth jerk free ride, with reasonable accuracy, and reasonable disturbance rejection.

It's one of the things which I would quite like to retro-fit to the W124 - some models in the range had it, but ours doesn't :(



Cruise control and fuel economy - Lud
The speed control of
most drivers is much poorer than even a crude cruise control. - typically holding the pedal almost
still and then being caught out if the wind or gradient changes a bit.


Quite a lot of drivers, perhaps a quarter, are far, far worse than you imply, NC. Constant random speed variation, often over a wide speed range, sometimes even involving the brakes for no earthly reason, is one of the infallible signs of a dangerously carp driver.
Cruise control and fuel economy - davidh
my old Mondeo
mk2 2.5 V6 auto was fantastic on CC - don't know if it was anything
to do with Ford using windows based system on engine management or not.


.

If I see a ford in my rear view mirror should I get out of its way?
Cruise control and fuel economy - henry k
>>This cruise control malarkey is new to me.
The Volvo is the first car I've owned to have it, and I am finding that I use it wherever the traffic conditions allow on the motorway.

>>
IIRC ( I read it on this site) it is illegal to use CC in the wet on motorways in Belgium.
I can only assume they consider it so unsafe in these conditions that thay have introduced a ban

My Mondeo II when in CC ignores any handbrake application. I think this is a bad design and SWMBO ( a non driver) knows how to disable the CC from the passenger seat.

Cruise control and fuel economy - Big Bird
Cruise control almost always gives better econmomy for me, but mainly becasue it stops me slowly creeping faster and faster. Saves NIP costs too.

If you compare like for like speeds it only really wins on flat low traffic motorways (like in northern France on the way to skiing, only 2 weeks to go ;-)

Dan
Cruise control and fuel economy - Alby Back
I know - It's really hard work on roundabouts, you have to time it just right ! ;-)
Cruise control and fuel economy - Bill Black
Up to about 60-ish the CC on my C5 was prone to immediately downshifting and then upshifting straight afterwards when turned on. When overtaking, because the auto kickdown was almost dangerously slow, I used to set the CC for about 80, turn it off and then turn it on again when I wanted a nifty pass, it worked a treat every time. Changing down manually was even slower! The CC wasn't particularly clever at maintaining a constant speed either, the speedo needle usually waved around + or - 2 or 3 mph.
Needless to say the CR-V is rock solid constant and the kickdown works as it should.
C5/CC = ~5mpg more and the CR-V/CC = ~5mpg less. Mainly A road conditions.
Bill

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