do you use more fuel when you have the cruise control on ??ie,motorway trips etc not sure myself,what do you guys think and do you use it often?

atb

michael

Edited 21 May, 201214 yr by freewheeler

Not sure if it uses more fuel but I use cruise control when ever it is practical, my last car did not have it but my next car will (whenever/whatever that may be).

I use it a lot - especially where there are speed restrictions on clear roads - then I can concentrate on driving rather than my max speed. It drives more aggressively than I do on open undulating roads so without any proof I suspect it is not as economical as me being in control of the accelerator in those circumstances.

I use it regularly on motorways and dual carriageways when there's not too much traffic about (otherwise I'm always having to switch it on and off as others change lanes, brake etc). I've travelled hundreds of miles across France where the motorways are better/emptier with it turned on. I've not noticed any drop off in fuel economy - if anything just the opposite!

Same opinion as Yetiflute: the speed changes are more sudden therefore not as economical. I've checked the same route under same conditions/traffic and cruise loses out. I do still use it in road works etc however.

: the speed changes are more sudden

Eh? What speed changes? The whole point is to turn it on and leave it - the speed doesn't change!

If you are careful with the throttle, you can easily beat cruise control for economy in hilly areas, by gaining plenty of speed on the downhills and letting speed fall away uphill.

On level and unobstructed roads there's nothing between cruise control and a driver who is aware of what they are doing with the throttle. On the other hand, there is a member of my family who, despite having driven more than half a million miles and being a good driver in every way that matters, cannot stop themselves constantly making small adjustments to the throttle, even on level roads. The cruise control beats them for economy on such roads.

I was sure it was more economical using cc, sure I read it was more efficient to accelerate quickly to desired speed than slowly, hence why the system works as such. My last car an X5, it was recommended in handbook as such.

I'm confused. I accelerate using the pedal to the speed I want to travel at (and the rate of acceleration depends on the road/traffic conditions) and once I'm there flick the cruise control on and leave it alone thereafter. I don't adjust the speed - just CRUISE at the chosen speed.

If you are careful with the throttle, you can easily beat cruise control for economy in hilly areas, by gaining plenty of speed on the downhills and letting speed fall away uphill.

And p1ssing off any following drivers? As well as potentially breaking the speed limit on the downhills...

If you have a speed stored in the cc and have to brake, press RES (resume) the car will accelerate back to stored speed.

And p1ssing off any following drivers? As well as potentially breaking the speed limit on the downhills...

Is is really necessary to add 'WHEN SAFE AND SENSIBLE TO DO SO' to every comment one makes about driving style? Or can we just TAKE THAT AS READ in future?

Sheesh.... It's not often I feel impelled to use capitals...

I was sure it was more economical using cc, sure I read it was more efficient to accelerate quickly to desired speed than slowly, hence why the system works as such. My last car an X5, it was recommended in handbook as such.

Yes, but that recommendation was about accelerating away from rest and up to cruising speed. Whereas people here are talking about the increase in speed triggered by the 'resume' button of a cruise control - a different thing entirely, which remains in one gear and opens the throttle neither a little nor all the way.

The point in your X5 handbook is just another way of saying get into the highest gear as soon as possible. Being in a high gear overrides all else when it come to economy. You can test this yourself by climbing the same hill twice at the same speed. The hill needs to be just the right steepness, where sustaining a steady speed in top gear requires you to have the pedal to the floor. Do it that way - top gear, pedal to the floor, no speed gain or loss - and watch the instantaneous consumption (or better, record it direct from the engine management system with an OBDII device.) Then do it again one gear lower, same steady speed; this time, of course, you will not need to put foot to the floor. This time the fuel consumption will be greater (i.e. worse). That is, being able to use a higher gear makes for better economy even if it requires a wide-open throttle. Some people hotly deny this, having the deeply-ingrained and over-simple idea that a wide-open throttle is bad for economy. Well, it is - but not in all circumstances; and the alternatives, such as changing down, may be worse.

LJK Setright won every fuel economy rally he entered by a very large margin. What few could understand was that he also went much faster than all the other competitors, reaching the finish half an hour before the pack and still beating them hands down on economy. One of his rules, based on a proper understanding of thermal efficiency of heat engines such as those in cars, was that acceleration is obviously expensive in fuel, but that when it must be done, it should be done on the best possible terms - foot to the floor, since all machines minimise their mechanical losses when working flat out. Nobody believed he could win economy runs with his foot to the floor at times, but he did it again and again. Other than 'foot to the floor', I'm not minded to go into all his methods here, because it would only provoke a discussion about whether it is SAFE AND SENSIBLE to drive thus. But his methods do work. I use them to get 32 mpg out of a 5-litre engine when I'm inclined to do so.

The resume function on a cruise control does not go through the gears and it does not accelerate hard enough to be relevant to this.

Edited 21 May, 201214 yr by r999

Well said 999.

motorways are better/emptier with it turned on.

Hey - MAGIC. You turn on your Cruise Control, DogBob, and hey presto! the roads are cleared.

You aint confusing your cruise control with a cruise missile, are you?

I can't see how CC can use more fuel on a steady rise/fall or level roads. What I have found (and it was worse on my last car than the Yeti) is that the CC can be slow to react at the bottom of a fall in an undulating stretch of road so the cars speed drops as you hit the next incline then it accelerates hard up the hill to regain speed - sometimes overdoing it by the crest of the next climb. In these circumstances you can beat the CC by careful use of the throttle.

...Being in a high gear overrides all else when it come to economy. You can test this yourself by climbing the same hill twice at the same speed. The hill needs to be just the right steepness, where sustaining a steady speed in top gear requires you to have the pedal to the floor. Do it that way - top gear, pedal to the floor, no speed gain or loss - and watch the instantaneous consumption (or better, record it direct from the engine management system with an OBDII device.) Then do it again one gear lower, same steady speed; this time, of course, you will not need to put foot to the floor. This time the fuel consumption will be greater (i.e. worse). That is, being able to use a higher gear makes for better economy even if it requires a wide-open throttle. Some people hotly deny this, having the deeply-ingrained and over-simple idea that a wide-open throttle is bad for economy. Well, it is - but not in all circumstances; and the alternatives, such as changing down, may be worse.

1. I don't *hotly* deny this, but anecdotal evidence shows driving at ~2K rpm on my diesel is most efficient up hills / under load... where the turbo starts being most effective, torque output maximised...

2. Of course diesel engines don't have "throttles" technically speaking, yet if I recall the theory on "throttle wide open being most efficient" related to carburetor-fed engines... not sure it applies to computerised fuel injection systems at all?

3. Interesting reference to LJKS - I was fascinated (albeit not a fan) by his writings in Car in the 1980s. Some of his books seem v difficult to get these days.... not many erudite motoring journalists around these days either.

Edited 22 May, 201214 yr by Totally Square

Yes, but that recommendation was about accelerating away from rest and up to cruising speed. Whereas people here are talking about the increase in speed triggered by the 'resume' button of a cruise control - a different thing entirely, which remains in one gear and opens the throttle neither a little nor all the way.

''Accelelerating away from rest'' not sure where this comes from... I didn't suggest it, I wouldn't think cc would get used by many away from a standing start. The BMW handbook certainly never indicated this either. The X5 manual was for automatic box, with a manual it's down to the driver to be in the correct gear to optimize acceleration back to set speed. To slow from say 70 mph in 6th down to 30mph then go with RES still in 6th gear will not get you back to 70mph as quick as going through the gears. My point being, that it is more efficient over a distance to accelarate to the desired constant speed quickly.

The point in your X5 handbook is just another way of saying get into the highest gear as soon as possible. Being in a high gear overrides all else when it come to economy. You can test this yourself by climbing the same hill twice at the same speed. The hill needs to be just the right steepness, where sustaining a steady speed in top gear requires you to have the pedal to the floor. Do it that way - top gear, pedal to the floor, no speed gain or loss - and watch the instantaneous consumption (or better, record it direct from the engine management system with an OBDII device.) Then do it again one gear lower, same steady speed; this time, of course, you will not need to put foot to the floor. This time the fuel consumption will be greater (i.e. worse). That is, being able to use a higher gear makes for better economy even if it requires a wide-open throttle. Some people hotly deny this, having the deeply-ingrained and over-simple idea that a wide-open throttle is bad for economy. Well, it is - but not in all circumstances; and the alternatives, such as changing down, may be worse.

I'm confused by your statement, perhaps you are too..

''a wide-open throttle is bad for economy. Well, it is - but not in all circumstances; and the alternatives, such as changing down, may be worse.''

A wide open throttle (perhaps before injection systems) is bad for economy or it isn't, changing down, may be worse or it isn't.''

Hardly definitive.

Edited 22 May, 201214 yr by servicepoint

I do a lot of miles and I always achieve a higher average on the maxi dot when I use my foot on the throttle, regular trips from the north west to Dartford about 230 miles each way, I have tried cc as much as possible going and just my foot coming back and vice versa, always cc gives a lower average mpg.

A wide open throttle (perhaps before injection systems) is bad for economy or it isn't, changing down, may be worse or it isn't.'

Measured over a few seconds, what you say is obviously correct. Most people, though, have a rather more sensible definition of fuel economy: how much fuel is used over a whole journey, or how much distance can be covered with a whole tankful, for example. And in that context, your statement is nonsense. It would be true only if speeds, terrain, circumstances, traffic never changed and the driver had to do nothing except obey one simple rule.

But you believe that a wide-open throttle is either bad for economy or it isn't. Go on, then, tell us: which is it?

r999

what a fail.

The statement in red is your statement from your earlier post.

So you explain it.

To my thinking an average driver is unlikely to control the 'throttle' as smoothly as the latest cruise control.

To qualify that statement...

An average driver would struggle to maintain a precise constant speed even if they were staring at the speedo all the time.

This means that they would be either accelerating (uses more fuel) or decelerating, in waves, which is a less economical driving style.

Cruise control makes much finer adjustments to maintain the speed more accurately, as it can react more quickly to changes, thus saving fuel due to much less 'wave effect'

CC systems technology is advancing too.

Our Fabia has it and it does take some time to react to that undulating road mentioned above.

Couple that with the fact that it will accelerate slightly more than needed going up hill and is less able to keep the speed in check going downhill and I can see where a skilled driver can beat CC in terms of economy.

Compare that to the system on our Yeti. It reacts much quicker and maintains the set speed much more accurately.

I think that a driver of the same skill level would struggle to win that battle.

My experience of it, which I admit only consists of a drive to Somerset and back, showed the CC to yield a slightly lower MPG than relying on my right foot. It is, however, more comfortable to be able to take your foot off the pedal, cross your legs, get into the back to have a kip...

I jest of course

I think on the emptier roads towards Scotchland, it'll be a real blessing, and I'll take the benefit of being more comfortable over the better MPG for a long drive like that.

I think Mercedes might have answered this one:

http://www.wired.com...ills-save-fuel/

On their Actros truck. Standard cruise control (as fitted to our Yetis) is NOT more efficient in hilly terrain - in fact it uses more than a human would... Hence Mercedes have now combined the sat-nav with the cruise control to help it pre-empt hills (and thus the correct gear to be in like a human would - or could if he/she was really concentrating 100% of the time) and Bob's your uncle: the truck SAVES 3% in fuel.

Hey - MAGIC. You turn on your Cruise Control, DogBob, and hey presto! the roads are cleared.

You aint confusing your cruise control with a cruise missile, are you?

Are you a journalist on "The Sun"? That's precisely the sort of selective quotation and hence misrepresentation that they indulge in.