cruise control fuel consumption figures

[johnfranklyn]

Has anyone tested cruise control against careful accelerator control over a fixed distance with regard to fuel consumption? I have tried it myself using the average mileage consumption figures and much to my surprise, find the difference so small it’s hard to establish any figures. Even on our local empty roads I had to drop out of cruise control at times, but not enough to make a big difference. I know the forum has its fuel consumption experts who may have done a more careful test than mine and wonder if they can inform me how big the difference actually is?

[SWCeed]

The official line is any driver can get better mpg not using cruise control, reason given is the driver can anticipate gradients, traffic etc and lift off earlier than cruise would.

However, in the real world that most of us inhabit I have found that it makes no difference, yes I can anticipate the road ahead better but I cannot keep up the constant speed that cruise control can and any variation of speed wastes fuel either when I overspeed or have to accelerate back to the speed limit..

Even if it did use extra fuel it would be worth it, after a 400+ mile holiday run mostly on motorways I still feel fresh which is down to having one thing less to concentrate on i.e. keeping to a constant speed.

[deeps]

I agree with SWCeed, In the real world there is little difference in fuel consumption. I would use CC even if there was a consumption penalty, and would not buy a car that did not have CC fitted. My car averages 50MPG at a steady 70MPH (satnav speed, 75 on the spedometer). I live in a light traffic area with motorways.

[Ace Demon]

Originally Posted by johnfranklyn

Has anyone tested cruise control against careful accelerator control over a fixed distance with regard to fuel consumption?

John, you obviously know by now that this sort of question has a Pavlovian response on me. No, I don't have cruise control...

I know the forum has its fuel consumption experts

...or bluffers?

who may have done a more careful test than mine and wonder if they can inform me how big the difference actually is?

On any such comparison, the first place to start is to perform a sanity check to see what might be achievable.

Theoretically, fuel consumed at speed is primarily dependant on aerodynamics so I will work on the known principle that drag is a function of the square of the speed.

So let's say, two cars do a 120 mile journey at an average speed of 60mph, and the cruise control car uses 2 gallons of fuel.

If the driver in the other car does 66mph for 60 miles and 55mph for the other 60 miles, the fuel used is (66/60)^2 + (55/60)^2 or 1.121+0.84 = 2.05 gallons (+2.5%). (Although the driver would normally oscillate around 60mph due to inclines and foot fatigue, for simplicity assume two speeds.)

So I suggest that any difference inherent in the car itself is just too far below experimental accuracy to identify. A person doing the experiment over two journeys would not be able to separate out the effects of other traffic, wind & weather and fuel variation. And is their driving genuinely consistent, other than the ability to maintain steady speed?

[Dr Bob]

I've thought about this a bit, but never tested it. As Ace Demon has calculated, the differences will be small.

I do note that some CCs are better than others. There is one section of highway that I regularly travel which is hilly with a speed limit posted at 90km/h. I wouldn't normally use CC on that road, but in one test, my car varied between 85 and 100 km/h because the CC seemed to be rather "lazy", and I'm sure that the fuel consumption was adversely affected. A previous car with manual transmission and more power would have kept a more constant speed, because the CC was more "aggressive". Somewhere in between (e.g. careful driver with no CC would be better for fuel consumption on this section).

On most flat and straight highways, I wouldn't be without CC.

[Spencernj]

I never considered the role of CC as being to save fuel. However, I find that practically CC does save me fuel as without CC on long motorway drives my speed creeps up. So although I intend to do 70mph suddenly find that I am over 80mph. With CC this does not happen. Given I travel about 35,000 miles a year even small differences matter.

Sent from my ZTE-BLADE using AutoGuide App

[johnfranklyn]

Thanks Gents for the replies. It seems CC is more efficient than I had imagined. It’s a bit of a culture type shock as I expected, with anticipation, to beat the CC hands down. To read my peers quoting my efforts would not make much difference has confirmed my dodgy road test. I will use it more often now. Trundling along with my caravan it is a Godsend I would hate to be without. Sorry RT

[derek c]

I have tested CC on four cars and could see no differance in MPG even I have been told by one Agent that it does.
I use CC about 90% of the time now even if I am going through a town I lock it on at 30 MPH
John, do you know that if you turn it off CC by the switch or touching the foot brake, it will go back to the original speed if you push the CC switch up

[SWCeed]

Originally Posted by derek c

even if I am going through a town I lock it on at 30 MPH

That is a very dangerous practice, CC should not be used in town. Suggest you stop it immediately.

Was not even aware CC worked at such a speed and not about to try it.

[derek c]

Thanks SWCeed, I have driven millions of miles in all types of vehicles ( hgv's & buses ) with no accidents and take a driving test every four years, so I think I am capable. Its like driving at 70 MPH you get used to it and I have driven over twice that speed up the M1