roof box and cruise control

Honest John Back Room » Motoring discussion
View Threaded
Wed 14 Apr 2004 15:21
roof box and cruise control - gadgetwilson
what difference if any will driving with a roof box and or cruise control make to MPG at motorway speeds.
Reply | Report as offensive | Link
Wed 14 Apr 2004 15:24
roof box and cruise control - No Do$h
I get a 15-20% drop in MPG with a roofbox on the car. What's cruise control? Ok, the last one was TIC.

HJ had an Avensis on test and seemed to get a noticable difference in MPG with cruise swithced on. Resulted in quite a debate. Try the "Forum Search" to the top right of the page, just below your log-in details.
No Dosh
mailto:Alan_moderator@honestjohn.co.uk
Reply | Report as offensive | Link
Wed 14 Apr 2004 17:44
roof box and cruise control - Honestjohn
The reason for my lousy economy on cruise might have been cruise itself, might have been a bad fill of Shell Formula diesel, might have been a headwind, or might have simply been driving too quickly. The biggest problem with roof boxes is corners. I run away and hide whenever I see a Discovery with one on top.

HJ
Reply | Report as offensive | Link
Wed 14 Apr 2004 17:48
roof box and cruise control - No Do$h
Although it's usually safe to come out again once the Disco is on top of the roof box, provided the noise has stopped by then.

:o)
Reply | Report as offensive | Link
Wed 14 Apr 2004 17:53
roof box and cruise control - Mapmaker
So what is a 'bad fill' that causes a car to reduce its mpg by almost one third?

www.honestjohn.co.uk/forum/post/index.htm?t=18694&...f

I'm struggling to understand what chemical difference might be in the fuel that leads to such a huge difference in petrol consumption. Is it one third water (see recent 'supermarket fuel' thread)? If not, why do you think this happens.

More likely that the fill was bad - i.e. that it didn't get filled up to the top of the tank, surely.
Reply | Report as offensive | Link
Wed 14 Apr 2004 19:06
roof box and cruise control - Honestjohn
The Cetane content of the diesel affects performance and economy.

HJ
Reply | Report as offensive | Link
Wed 14 Apr 2004 20:11
roof box and cruise control - gasoil
Perhaps it was a bad batch of diesel? where did you get it???????? have you tried the ultimate stuff from bp?
Reply | Report as offensive | Link
Thu 15 Apr 2004 11:24
roof box and cruise control - Mapmaker
Thanks, I understand that. I just refuse to believe that a particular tank of diesel would be so impaired as to offer 30% less mpg. Surely the car would run as rough as anything - if at all!
Reply | Report as offensive | Link
Sat 17 Apr 2004 02:17
roof box and cruise control - MichaelR
Can cruise control reduce economy? I always thought it would increase economy, becuase it can maintain a constant speed a lot better than the average foot.
Reply | Report as offensive | Link
Sat 17 Apr 2004 08:34
roof box and cruise control - Honestjohn
I don't know abut averaage feet, but cruise control is a lot less sensitive than mine. And it does what you would not do. For example, if you're cruising at 80 and you reach an upward incline, rather than step harder on the gas you will probably allow the car to settle back to maybe 75 or 70. But cruise will just pour however much source it needs to maintain 80.

HJ
Reply | Report as offensive | Link
Sat 17 Apr 2004 09:10
roof box and cruise control - Ben79
Cruise control does what it says on the tin, keeps a constant speed, uphills or downhills. I don't find many motorways where lifting off the gas results in an increase in speed downhill.

You could decrease the speed using the buttons when travelling up motorway hills, but on the M62, I find that by doing 70ish up the hills, you overtake all the eejits who do 80 or 90 down the hills.

Also, why are we saying "gas", have we turned American, or has HJ got an LPG conversion?
Reply | Report as offensive | Link
Sat 17 Apr 2004 15:47
roof box and cruise control - gasoil
Have you tried filling the roofbox with 5 gallon drums of kerosene or cooking oil, this does wonders for the fuel consumption!
Reply | Report as offensive | Link
Sun 18 Apr 2004 01:08
roof box and cruise control - JohnM{P}
My current Golf 110TDi is my first car with cruise control, and I am convinced that using it uses more fuel than without it. My assumption is the same as HJ's, that I tend to drive more at constant accelerator opening rather than at constant speed. I have tried to verify this conviction, but have found that fuel consumption varies significantly day to day over the same (long) stretch of motorway at what is seemingly the same set speed. I reckon that wind and wet roads play a significant part in these variations.

Regarding topboxes and mpg, my one European trip with a topbox on a Passat 110TDi (which normally showed 52mpg ish on trip computer) started frighteningly with a dash up the M4/M25/M2 to catch the ferry giving sub 40mpg!. More sensible speeds on motorways through France, Belgium to Germany and back recovered the mpg figures to 44mpg ish, if I remember correctly...
Reply | Report as offensive | Link