Cruise Control & MPG

[Timewarp]

Hi,

I notice if I have the instant MPG readout on on the vRS and I'm going down a hill without accelerating the readout shows ---.

However, if I have cruise control activated and I'm going down hill the readout shows a high MPG number (e.g. 60MPG +).

I just wondered, does this mean more fuel is being used if I've got cruise on downhill?

I thought modern cars are meant to use no fuel at all if you're moving with the clutch engaged and are applying no throttle.

Ta!

[TommyC]

The cruise control will be applying some throttle to prevent the car decelerating (it will still decelerate, just at a slower rate than on the flat, due to aerodynamic drag, tyre drag etc etc etc).

AFAIK the trip computer counts injector pluse and duration to calculate the mpg as accurately as possible.

[chris22]

I was wondering exactly the sameabout using more mpg with cruise on.
Some people say it uses less fuel with it on because it can keep a consant speed better than you can, others say it uses more fuel because it doesnt slow down at all on hills.
From my limited experience with it (only had the car a few weeks) I have found it uses a little bit more fuel with cruise on.

[jwruk]

I've had cruise fitted for a week now, and on my daily commute (25 mile m/way round trip) I've found the average mpg to be upto 5mpg lower if I'm using cruise.

[Lady Elanore]

I defiantly notice a drop in MPG with cruise. I put it down to the fact that cruise can’t predict the changing road conditions or other drivers behaviour.

[myfatcow]

all cruse is doing is holding your revs so depending on what gear you are in depends on your revs i have cruse i only use it when im in a lazy driving mood or fancy a sleep on the way home from work. if you want to save your fuel dont use it if you are lazy like me use it. Remember you car dosent care how much fuel you are using! it just wants to keep to the speed you tell it to.

[TommyC]

I drive 2 long journeys per week approx 145miles each. Before i had cruise i'd struggle to get above 50mpg. Since having cruise fitted by Stu my mpg at the end of each journey is easily above 50 (and has increased since i started using Millers sport 4). I keep the cruise on as much as i can and usually set it at 80. The motorways i cover are M25 between jct 5 & jct 16 and M40 up to jct 8. There are some pretty big hills on both but they have little overall effect because what you burn climbing them, you save on the descent. I do choose to travel late evening after rush hour (because i can) and rarely have to turn cruise off once it's on. The problem with it is that when you press resume it accelerates quite hard to get to the speed it wants to be at. I accelerate more gently to around the right speed and then hit resume.

[TomH]

Originally Posted by Lady Elanore

I defiantly notice a drop in MPG with cruise. I put it down to the fact that cruise can’t predict the changing road conditions or other drivers behaviour.

Yes, meaning you have to brake rather than back off which wastes fuel!

[Lady Elanore]

Originally Posted by TomH

Yes, meaning you have to brake rather than back off which wastes fuel!

[myfatcow]

Originally Posted by TommyC

I drive 2 long journeys per week approx 145miles each. Before i had cruise i'd struggle to get above 50mpg. Since having cruise fitted by Stu my mpg at the end of each journey is easily above 50 (and has increased since i started using Millers sport 4). I keep the cruise on as much as i can and usually set it at 80. The motorways i cover are M25 between jct 5 & jct 16 and M40 up to jct 8. There are some pretty big hills on both but they have little overall effect because what you burn climbing them, you save on the descent. I do choose to travel late evening after rush hour (because i can) and rarely have to turn cruise off once it's on. The problem with it is that when you press resume it accelerates quite hard to get to the speed it wants to be at. I accelerate more gently to around the right speed and then hit resume.

how do you manage to get 80mph on the m25 when i drive on it i am normaly doing about 20mph in the fast lane. worst road in the UK

[Timewarp]

Originally Posted by TomH

Yes, meaning you have to brake rather than back off which wastes fuel!

You can just flick it off can't you?

Or if you brake very lightly it turns off cruise without the calipers gripping.

[TomH]

I don't know... I've not even tried it yet!

[TommyC]

Originally Posted by myfatcow

how do you manage to get 80mph on the m25 when i drive on it i am normaly doing about 20mph in the fast lane. worst road in the UK

I drove round the M25 last thrsday night at 11pm and only had to switch off cruise once!!

I choose to travel late evening after rush hour (because i can)......

I also travelled back on sunday night at 10pm. Only switched it off 3 times.

It just that i have the oportunity to travel at those times and not sit in traffic. I spent a few years doing that and now decide not to......

[Mebuyaskoda!]

I was wondering about the cruise control and MPG. I assume that when going downhill with the cruise control on it will keep the revs going at whatever speed you have set the car to travel at. Would I be right in thinking that without it on you would invariably take the foot off the gas pedal to maintan the speed you wanted thus the engine revs would be lowered and therefore burn less fuel?
saying that I was very naughty the other day.. I was behind an admittedly older Escort RS with all the go faster bits on I wasn't bothering much but he pulled out to overtake a car that was having a struggle climbing the Rest and be Thankful (i'm on this road every w/end) anyway I thought "ok I'll just follow you mate" well he shot off up the hill with me in pursuit not even trying was I! well his passenger kept turning round to see if I was still there .. I was and then suddenly alsorts of black smoke started appearing from his car... I feel bad about that now.. Hope he got home ok!! Sorry for going of at a tangent but the reason I mention it is because I still got over 60MPG on that 100 run!

[chris22]

If you keep a constant speed engine revs will stay exactly the same whether using cruise or not.
But I understand that if you go downhill (or just coast) and take you foot off the gas then the engine does not use any fuel. (I used to have a petrol astra and the manual said exactly that)
Also that trip computer is overestimating a hell of a lot.

[Stuart_J]

Im suspecting the problem with cruise is that when you come to an upward incline it throws a load of fuel in quickly to maintain the speed wheras you would tend to gently squeeze the pedal. Either way I would agree that it uses more fuel on cruise.
Years ago they said the most economical way to drive was to allow the speed to slowly rise on downhill stretches & then slowly fall back going up hill, some people also recomended knocking the car into neutral going down hill to remove the engine braking effect, personally I wouldnt bother with that as I feel the car is less stable when out of gear if you suddenly need to react to something.
The old V4 Saabs were equiped with free wheeling hubs to generate this very effect.

[TommyC]

But Cruise only uses what it needs to maintain revs. It's a computer. It checks the sensors etc more times per second than your brain can. By the time you realise you're dropping below the speed you want and press the accelerator to speed up, it will have checked the system and made hundreds of minor ajustments to maintain speed. It will have used less fuel to maintain speed than you will have to accelerate back to the speed you want.

It's like which is quicker at processing information, a BBC micro or a Pentium 4.......

[Zarqwan]

to resurrect an old thread...anybody else yet found a better MPG with cruise control. Am thinking of having it fitted. Should I?

[Fabia_TDI]

Well here's my experience. Without cruise control I tried to stick to 70 but usually failed, tending to pick up speed when overtaking on dual carriage ways and not slowing down again, possibly because I was subconsciously wanting to get home and have a cup of tea.

With cruise it seems to be easier - the journey might take a fraction longer but I stick to 70, it's far less effort to drive and under those circumstances I would save fuel (all important these days).

Whether or not you should get it depends on the cost/hastle. I like it though!

[Lady Elanore]

I find it is less economical to use cruise, but it is great for road works on the motorway when there are a lot speed cameras about.

Cruise can also be a pain when you find a car that wants to travel around the same speed as yourself but isn’t using cruise. You will find the other driver forever overtaking you and then you overtaking them. It’s a pain. Having said that I paid extra to have it on my car, so I guess I must really like it overall.