Fuel efficiency and satellite domes

A thought occurred to me about the sat dome on our Broadway. It is the standard size dome but I remember reading advice from the AA some time ago about roof racks and car fuel efficiency. The advice was where possible to remove the rack when not in use because it does lower the fuel efficiency (upto 25% was quoted in one article I read recently!). Now looking at our sat dome it does seem to be something that could increase fuel consumption because, to put it mildy, it is a big lump of a thing stuck onto the roof. Even roof boxes are now much more ergonomically designed. I was just wondering if anybody has found any information on this topic. Or is it not something to be bothered about?  (BTW one of the reasons we went for the low profile version of the Broadway was because of reported fuel efficiency due to the roof design. It's be a shame of the sat dome undid all this.)

I suppose that this is the case with all protrusions on the top of a vehicle. We have a Snipe dish, solar panel, TV aerial, and roof bars - and I'm wondering why we only average 27mpg.

P.S. Did you get your remote yet? Does it work now?

meanchris wrote:I suppose that this is the case with all protrusions on the top of a vehicle. We have a Snipe dish, solar panel, TV aerial, and roof bars - and I'm wondering why we only average 27mpg.

P.S. Did you get your remote yet? Does it work now?

Still waiting for the remote. It's somewhere between Germany and Ireland apparently. Should be here in the next day or two then I'll have a go and post updates. Thanks again.

I'd agree with meanchris, in that it must have some effect. However, IIRC around a year or so ago one of the motoring mags tried to reproduce the AA's "up to 25%" claims (for an unloaded roof rack) and could not get anywhere close to that figure. IIRC their conclusion was that you could expect an MPG hit of "a few percent", and the sat dish is probably around the same, IMO.

No doubt somewhere along the graph slope there will be a 25% increase in fuel consumption, but I bet it'll be at about 150mph...
Al.

bikeralw wrote:No doubt somewhere along the graph slope there will be a 25% increase in fuel consumption, but I bet it'll be at about 150mph...
Al.

A headwind of 2 or 3 mph, or an increase in speed of about the same, will have a far greater effect on mpg than a smooth and fairly aerodynamic dome.

Your door mirrors will present a greater wind resistance, yet nobody even gives them a thought!!

Gromit wrote:A headwind of 2 or 3 mph, or an increase in speed of about the same, will have a far greater effect on mpg than a smooth and fairly aerodynamic dome.

Your door mirrors will present a greater wind resistance, yet nobody even gives them a thought!!

I do, Sir, me me me, I do.

Swot!!  

I can't understand those who are paranoid about fuel consumption. If one wants to go on holiday it will require a certain quantity of fuel, so one may as well fork out and stop worrying about it. Or stay at home. It's one part of the holiday cost that can't be avoided, so fretting about it can't help at all.

Having said that, I'm equally perplexed when a big motorhome flashes past us doing at least 80mph on the motorway, which happens fairly often. I sometimes wonder if they are the same ones who complain that their van will only do 20 to the gallon! 

The old advice is still good - stay with the lorries at around 55mph to get (arguably) the best compromise w.r.t. fuel economy.

Gromit wrote:Swot!!  

I can't understand those who are paranoid about fuel consumption. If one wants to go on holiday it will require a certain quantity of fuel, so one may as well fork out and stop worrying about it. Or stay at home. It's one part of the holiday cost that can't be avoided, so fretting about it can't help at all.

Having said that, I'm equally perplexed when a big motorhome flashes past us doing at least 80mph on the motorway, which happens fairly often. I sometimes wonder if they are the same ones who complain that their van will only do 20 to the gallon! 

The old advice is still good - stay with the lorries at around 55mph to get (arguably) the best compromise w.r.t. fuel economy.

And best of luck when you are squeezed in between 2 lorries on the French toll roads, who leave less than a metre between you and them!  Also applies in this country by the way.

I do totally agree with you on stop thinking about fuel, just get on with it and enjoy your holiday.
----------
Richard

you will probably find that the deflection of air off the front of the vehicle mostly shields the sat dome.  The roof area is mostly turbulent rather than laminar air flow so probably has a lot less impact than you would think.  Car's are more aero so roof mounted kit is going to have a greater impact.

The domes I have seen have worse aerodynamics than a brick wall. I would expe g at least a 5% penalty on Nov. But does it matter?

Newer vans have better economy in my experience. Our friends have a 2.8 Fiat and get about 22mpg, we have a 2.3 150 Fiat that's several years newer and we get 31mpg. Although both are 3.5t theirs is working harder especially on hills where he uses lower gears. 
Rich.

Thanks for the replies. And some very interesting observations. I was trying to find out if any definitive research had been done on this particular issue. I am aware of the other points about speed, wind, wing mirrors, aircon. etc but I am just generally interested in minimising my fuel use as much as possible and this seemed an another thing to investigate. I wonder if the manufacturers of these domes have actually done any wind tunnel testing. I'll ask them and report back if I get anything useful.

PitStopCrew wrote:you will probably find that the deflection of air off the front of the vehicle mostly shields the sat dome.  The roof area is mostly turbulent rather than laminar air flow so probably has a lot less impact than you would think.  Car's are more aero so roof mounted kit is going to have a greater impact.

Very interesting. I hadn't thought about the turbulent air flow over the roof.

inspiredron wrote:The domes I have seen have worse aerodynamics than a brick wall. I would expe g at least a 5% penalty on Nov. But does it matter?

It matters to me. I guess that generally speaking I have always tried to minimise the fuel I use in all my vehicles. Maybe it's my upbringing or something

If you have a high van the dome can mean that it triggers the sensors at some tolls that you are a truck. On French roads pushing the attention button and telling the operator that you are a camping car usually will get the toll reduced, don't bother trying this at the Tyne Tunnel though, they couldn't care less.

meanchris wrote:

P.S. Did you get your remote yet? Does it work now?

Got the remote. Connected the sat receiver by HDMI to the TV and all works fine. Thanks for the help on this one.