View Full Version : Does it affect mpg?
Deleted account DD
13-01-2011, 22:42
I usually leave my roof bars (not the rails that run the length of the car theyre bolted on :D i mean the bars that go cross ways) on all the time, its handy and Im too lazy to take them off :nenau:D
Has any one doe the maths to work out just how much extra fuel you use if you leave roof bars or roof racks on I know it does , just not how much :nenau
zippy656
13-01-2011, 22:47
nope. but it MUST cost you something..
maybe 10 miles a tank full would be my GUESS>.
It will cause drag & noise maybe :nenau That is a FACT (not taking the P)
As for altering the MPG its a load of tosh
My mav when i 1st got it avaraged 300 miles on a full tank
My mav now with 33's / roof bars with 4 waffle boards / snorkel / lights . . .
Yep . . . . . Still 300 miles averaged :thumbs
zippy656
13-01-2011, 22:56
oooo tosh.. not used that for ages!!
what about the roof lights... that make any diference?
extreme-4x4
13-01-2011, 22:57
na zipps its wayyyyyyyyyy more scientific than that .... sadly im not a scientist .
there is however a point / speed where earodynamics becomes a massive problem its around 55 or 60 mph where your using more energy to push it through the wind and this wind pressure is getting worse all the time from then on . there is another point where is steps up massively , i think this is around the 150 mark . for cars that is and about as far as we need to know about.
but there are other large jumps too, further on. i remember reading the early jet planes had some big issues with these (unexplained at the time) jumps in force
thats the only help i can offer
zippy656
13-01-2011, 22:59
ah right, thats why my truck sits better at 55 / 60 than 150 mph then.. right??
still thinking about my roof lights...
extreme-4x4
13-01-2011, 23:00
It will cause drag & noise maybe :nenau That is a FACT (not taking the P)
As for altering the MPG its a load of tosh
My mav when i 1st got it avaraged 300 miles on a full tank
My mav now with 33's / roof bars with 4 waffle boards / snorkel / lights . . .
Yep . . . . . Still 300 miles averaged :thumbs
but you have bigger tyres on . so id expect better mpg probably balance it out and whats your top speed ?
If you put something solid like a roof box or tow a caravan for example then your causing drag :thumbs
Putting lights on your roof the wind will go through / round the gaps :thumbs
Are you telling me a roof ariel will cost you mpg :rolleyes: THINK.! :thumbs
Look at lorrys when your next on a major A road or Motorway :thumbs
On a vehicle where every penny counts towards economical fuelling :thumbs
If spot lights on there roofs (mine has them) affected mpg then they wouldn't put them there :thumbs
zippy656
13-01-2011, 23:07
convinced me there mate...
but you have bigger tyres on . so id expect better mpg probably balance it out and whats your top speed ?
300 miles when it was mint & 300 miles now...
90mph when it was mint & 90mph now...
Only difference is the time it takes to get there...
but you have bigger tyres on . so id expect better mpg probably balance it out and whats your top speed ?
bigger tyres, higher gearing, more fuel to pull away, optimum speed for fuel efficiency is around 56mph, i read this somewhere but cant find it at mo, :thumb2
BigBlack
13-01-2011, 23:11
Does it affect MPG...... simple answer.... yes.....
Increased drag = increased fuel consumption... (FACT! :lol ) ok, its only an inch thick but when its being pushed through the air at 60+
However, turning on your air-con, radio, blowers, lights, opening the window, carrying cr*p in the boot, driving into a head wind, driving up a hill etc etc etc etc will all affect your fuel consumption
In the grand scheme of things if you use them regular and can afford an extra quid or two a week leave them on :nenau
extreme-4x4
13-01-2011, 23:20
just one i found quickly . http://www.metrompg.com/posts/roof-racks.htm
not over interested in an argument over it .... i dont like lights or shite on the roof anyway.
makeitfit
13-01-2011, 23:21
Has any one doe the maths to work out just how much extra fuel you use if you leave roof bars or roof racks on I know it does , just not how much :nenau
NO :eek:
I do , surprisingly , have a life.:lol:lol:lol:lol:lol:lol
Oh and I'm too lazy to take mine off too:doh
(RIP) PLANK
13-01-2011, 23:30
Look at lorrys when your next on a major A road or Motorway :thumbs
:thumbs
sadly lorries aren't built for fuel efficency and aerodynamics at all! they are built for the largest volume of truck withing the legal maximum dimensions, that's why they have a slab flat front and the driver sits over the engine.
with uk and europe's laws if you had the engine out front (like a USA truck) you could not tow a full lenght trailer. It's all about load capacity not aerodynamics.
there was a Scania with a long bonnet marketed wonce and it made the whoel thing 2 feeet over length when towing. I could look the model and info up and provide a long, but i can't be arsed!
aerodynamics afects energy used at any speed, that's why runners, and cyclists pay it so much attention.
So yes you roof bars wil case drag, but how to work out how much fuel? i haveno idea.
but there are so many other variables, for example if a car travels 100 on 10 litres of fuel.
the milage will change according to:
the exact roads used
followig traffic
the trafic you are following
traffic on other side of the road
gradiants
side wind
head wind
tyre pressures
air temperature
air pressure
fuel composition and quality
etc. etc.
So if you need very accurate results you would have to run a test in controlled circumstances where all variables could be fixed, and then add or remove drag.
And that we can not do.
but it is a fact that anyhting you add to the car, will change fuel economy one way or the other, weather ou can mesure the change or not is another question.
sadly lorries aren't built for fuel efficency and aerodynamics at all! they are built for the largest volume of truck within the legal maximum dimensions, that's why they have a slab flat front and the driver sits over the engine.
I beg to differ when the Scania & Volvo bull nose's are on the road.!
(RIP) PLANK
13-01-2011, 23:41
I beg to differ when the Scania & Volvo bull nose's are on the road.!
beg all you like it's still true, they have to meet the size criterea, the trucks you mention are obvioulsy more aerodynamic, but the scania you quote i believe is the model that - with maximum lenght trailer - becomes over the limit, but scania decided to put it into procduction anyway.
I looked into this in depth recently when researching for a book (if you remember i started this by asking for hints and pointers on here, and you helped me out). I don't have tome to drag it all out now, but i do have it all in print.
I may have got the exact models wrong, but trucks are built for capacity not aerodynamics, of course it is a compromise, and there will be minor sacrifices to one or the other, hence the range and choice of trucks - but not much!
It was a trend started in by the Reanault Magnum, max size in a given space, the design saw off most of the competition and the rest followed suit. It's now an interesting piece of road haulage history.
beg all you like it's still true, they have to meet the size criterea, the trucks you mention are obvioulsy more aerodynamic, but the scania you quote i believe is the model that - with maximum lenght trailer - becomes over the limit, but scania decided to put it into procduction anyway.
I looked into this in depth recently when researching for a book (if you remember i started this by asking for hints and pointers on here, and you helped me out). I don't have tome to drag it all out now, but i do have it all in print.
I may have got the exact models wrong, but trucks are built for capacity not aerodynamics, of course it is a compromise, and there will be minor sacrifices to one or the other, hence the range and choice of trucks - but not much!
It was a trend started in by the Reanault Magnum, max size in a given space, the design saw off most of the competition and the rest followed suit. It's now an interesting piece of road haulage history.
Its not over length & doesn't need CAT 1 / 2 / 3 & is legal FACT
They way they got round it for the UK market was to only put the 4 wheel tractor units on sale in the UK & then Volvo followed suite.
You will see more bull nose recys & where there more commonly used as the Dutch do many & various axle conversions & the bull nose models in general had bigger engine options.
Thomas-the-Terrano2
13-01-2011, 23:53
ws under impression that in uk specs, not europe, it was an issue to haul
full size trailer with a yank tractor unit, but that scania is quite a short
bonnet and ive seen several hauling 40'ers, may 45's, look good but given
choice would keep the wheelbase of the tractor and have more sleeper
cab over engine, ie double bed plus behind seats. kenworth aerodyne style.
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ws under impression that in uk specs, not europe, it was an issue to haul
full size trailer with a yank tractor unit, but that scania is quite a short
bonnet and ive seen several hauling 40'ers, may 45's, look good but given
choice would keep the wheelbase of the tractor and have more sleeper
cab over engine, ie double bed plus behind seats. kenworth aerodyne style.
All flat cabs with i.e. Scania Topline are double bunked :thumbs
I usually leave my roof bars (not the rails that run the length of the car theyre bolted on :D i mean the bars that go cross ways) on all the time, its handy and Im too lazy to take them off :nenau:D
Has any one doe the maths to work out just how much extra fuel you use if you leave roof bars or roof racks on I know it does , just not how much :nenau
just one i found quickly . http://www.metrompg.com/posts/roof-racks.htm
not over interested in an argument over it .... i dont like lights or shite on the roof anyway.
Back on topic :augie
http://fuel-economy.co.uk/indepth1.shtml
extreme-4x4
14-01-2011, 00:01
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love it
zippy656
14-01-2011, 07:27
yep great..
Thomas-the-Terrano2
14-01-2011, 08:27
double horizontal, not vertical
extreme-4x4
14-01-2011, 10:20
It will cause drag & noise maybe :nenau That is a FACT (not taking the P)
As for altering the MPG its a load of tosh
My mav when i 1st got it avaraged 300 miles on a full tank
My mav now with 33's / roof bars with 4 waffle boards / snorkel / lights . . .
Yep . . . . . Still 300 miles averaged :thumbs
yes very :augie:augie:augie
the following taken from your own link
Its not just windows that will cause drag either! Listed below are some steps that will help reduce drag.
As mentioned above, when travelling at speed close your windows and sunroof.
If you are lucky enough to be driving a convertible in the summer, stop and put the roof up (provided its safe to do so) before you drive on motorways or fast A roads.
Remove any roof-racks, roof-boxes or bike racks when you aren't using them. These are some of the worst offenders when it comes to drag!
Its great to support for the national team, but remember to remove your England flag from your car when the matches are over.
If your car's bonnet is covered with dead flies and lumps of mud and goodness knows what else, wash your car! It may seem like a pretty pointless thing to mention, but all those extra lumps and rough edges will disrupt the airflow passing over your car, causing more drag. Oh and everyone likes to see a shiny motor!
I usually leave my roof bars (not the rails that run the length of the car theyre bolted on :D i mean the bars that go cross ways) on all the time, its handy and Im too lazy to take them off :nenau:D
Has any one doe the maths to work out just how much extra fuel you use if you leave roof bars or roof racks on I know it does , just not how much :nenau
It would depend on how aerodynamic the car was in the first place, cars designed to give the greatest fuel economy will be greatly affected by any thing that spoils the smooth flow of the air over the car.
The effect of drag is not linear, twice the speed doesn't create twice the drag, it can create 2x2 ie 4 times the drag. So aerodynamics is more critical at high speeds.
Landies which are as aerodynamic as a house brick, won't be greatly affected by 4 roof mounted spot lights at 50 mph.
However an economical low drag car will be greatly affected by even a window slightly open, and certainly a roof rack will cause turbulance, noise and heat caused by the friction, the energy has to come from some where, ie the fuel.
In a word, you will be using more fuel, how much as stated in the previous threads, depends on speed as well. :thumbs
What I never understood is that tin box towers used to have the roof mounted wind deflectors, that at the correct angle could save fuel at a particular speed. :nenau
If this was tested at say 60mph in a wind tunnel, drag could be reduced considerably, but they were fitted in the wrong place, the wrong angle, but worse than that, they drove around on their holiday with it still fitted to the car, burning all the fuel that they might have saved on the journey to and from the holiday location. :doh
As an example, we drive the Picasso on long journeys, and occasionally we tow a small 5x3 camping trailer, even with little weight in it, the fuel economy falls from 55 mpg to well below 50 mpg.
Caused by the turbulance and drag of the trailer.:doh
We try to fit everything into the car where possible.
To test out the drag, try putting your hand out of the window at 20 mph or even more...
WARNING: May cause injury, so take care.. :eek:
it will effect it but to say exactly how much is difficult.
But my opinion is surely it can't effect it much. Maybe 1mpg less?
BigBlack
14-01-2011, 13:54
Maybe 1mpg less?
To look at it a different way then...
10,000m @ 28mpg = 357.14g / 1623.59ltrs @ £1.30p/ltr = £2,110.67
10,000m @ 27mpg = 370.37g / 1683.73ltrs @ £1.30p/ltr = £2,188.84
So... extra £78.17 per year.... or roughly 60ltrs of fuel! :cool:
ps: ok, I know most dont get 28mpg.... but rough estimate! :thumb2
extreme-4x4
14-01-2011, 14:40
well, as you know mine is trimmed a lot but still got the original bumpers on just trimmed in the places i think best.
i recon with the bigger tyres only 31's at the mo i save around 3 mpg.
i dont have any data or facts / proof . but thats what i recon i save . and i suppose like everyone else trying to save a bit here and there .
i could be right and saving 3mpg or i could be wrong and blindly suffering some kind of placebo effect .... but unlike the people who do leave the roof bars on or loads of other crap in the way causing drag ..... i still get a warm feeling thinking im saving some dosh
(RIP) PLANK
14-01-2011, 15:29
Its not over length & doesn't need CAT 1 / 2 / 3 & is legal FACT
They way they got round it for the UK market was to only put the 4 wheel tractor units on sale in the UK & then Volvo followed suite.
You will see more bull nose recys & where there more commonly used as the Dutch do many & various axle conversions & the bull nose models in general had bigger engine options.
look, this is going way off course, i reffered to one specific scania model circa 1990, not current models, so your point is missing the point! The point is tractor units are not the most aerodynamic vehicles on the road and are designed for overall capacity not fuel efficency - that was my point and it is still very true.
For example, take a given truck and through an aerodynamic cab profile increase the fuel efficency by 10% and in doing so reduce its overall load capcity by 10% you now need 11 loads were there were 10 so 10% more trucks on the road and 10% more fuel used, not gains whatsoever.
in these new fuel efficent times economy will drive design, but only to an extent it will allways be a compromise;
my point in short:
Whatever you bolt to your roof will afect fuel economy - full stop.
and comparing it to a huge pantechnicon will not help!
Now lets get back on topic :thumbs
(RIP) PLANK
14-01-2011, 15:32
[QUOTE=Fez_uk;126406]it will effect it but to say exactly how much is difficult.
QUOTE]
spot on :thumb2
Deleted account DD
14-01-2011, 19:05
All flat cabs with i.e. Scania Topline are double bunked :thumbs
the one I was in yesterday wasnt :augie
Deleted account DD
14-01-2011, 19:10
Definitely getting less mpg on mine on a mixed driving cycle with the supposed "aerodynamic" roof bars fitted.
Theres absolutely no dount that anything that disrupts the airflow will affect consumption. May be so neglible you wont know it I suppose.
Thanks for the input, i posted the question before I logged off for the night and I cant believe the number of replies :lol:lol:lol
extreme-4x4
14-01-2011, 19:58
what you mean is you planted the seed before logging off, then left it for all the following inevitable disputes.
:augie:augie:augie
Deleted account DD
14-01-2011, 20:04
what you mean is you planted the seed before logging off, then left it for all the following inevitable disputes.
:augie:augie:augie
:augie
no wouldnt do that.
I just suspected there may have been someone had worked it out, I know it does affect it bt I just dont know how much by.
I was reading up on those kenlowe heaters and theres one bloke worked out using it saves him 2.7mpg during the winter htf did he work that out so precisely :nenau:D
The maximum lenth for a Lorry on uk road is 16.5 meters, that is why the bull nose lorry never caught on here. They did have a few running tippers as they can get the weight up to 44 tons.
A UK road train, classed as a wagon and drag is 18.5 meters.
Any how, this was about drag and fuel, YES the roof bars will increase you fuel consumption. I did say i was going to post tips on this but have not had the time to do it yet, as i want to make sure its right.
But here are a few:-
Dont let your car warm up before driving it, modern cars dont need it, just dont thrash it to death before its warm and its fine.
Reverse into your driveway or garage, because the engine is warm it will use less fuel for this manover now than reversing out in the morning when its cold.
Dont drive with your window open if its warm and you have aircon, Yes Aircon uses more fuel when its running than when its not, but having the windows open cause drag and that uses even more fuel.
When in heavy traffic or at traffic lights turn the engine off, BMW have add this to some of their cars as standard, they just cut out when stood, then dip the clutch and they start back up and drive away.
Just a few to be gettin on with, i will do a more detailed one soon.
Deleted account DD
15-01-2011, 18:15
When in heavy traffic or at traffic lights turn the engine off, BMW have add this to some of their cars as standard, they just cut out when stood, then dip the clutch and they start back up and drive away.
The missus's SEAT Leon does that , its quite spooky at first but it does save a massive amount of fuel :thumb2
And as she refuses to have roof bars fitted ...........................
Ya wind-up merchant :augie
Unless ya ars sqeeks while ya walk to your vehicle :augie As soon as you move its causing drag :rolleyes:
The more you put on or in its obvious its going to increase drag :augie
But i will stick to my words & say it doesnt make a difference, in the respect of it being noticeable to your fuel bill :rolleyes:
If you want fuel efficiently used then get a push bike :lol :thumbs
(RIP) PLANK
15-01-2011, 18:51
if i fill my tank and get 300 miles fomr it
then pot 33 inch tyres on and still get 300 miles
is it possible the milometer is now out of calibration due to the wheel size change, if the speedo reads diffreent then so will the milage? :augie
if i fill my tank and get 300 miles fomr it
then pot 33 inch tyres on and still get 300 miles
is it possible the milometer is now out of calibration due to the wheel size change, if the speedo reads diffreent then so will the milage? :augie
Not when I use GPS :augie
makeitfit
15-01-2011, 19:00
if i fill my tank and get 300 miles fomr it
then pot 33 inch tyres on and still get 300 miles
is it possible the milometer is now out of calibration due to the wheel size change, if the speedo reads diffreent then so will the milage? :augie
I worked it out on mine with 33s on the speedo UNDER reads by about 6-7% vs the sat nav.
I know with all my mods I'm still around 26 to the gallon:rolleyes:
Which I'm quite happy with as I've got lead soled boots and roof lights and 'king heavy winch and it runs on bio at 82p a litre :D
(RIP) PLANK
15-01-2011, 19:28
I worked it out on mine with 33s on the speedo UNDER reads by about 6-7% vs the sat nav.
I know with all my mods I'm still around 26 to the gallon:rolleyes:
Which I'm quite happy with as I've got lead soled boots and roof lights and 'king heavy winch and it runs on bio at 82p a litre :D
that's abut all i get running standard! so you must be doing somehting right :thumb2
I think the key phrase in all of this is about, GPS (with a mileometer? :augie) or not. There is no way to tell unless the difference is pretty large.
that's abut all i get running standard! so you must be doing somehting right :thumb2
I think the key phrase in all of this is about, GPS (with a mileometer? :augie) or not. There is no way to tell unless the difference is pretty large.
The bloody speedo don't read right even with the correct wheel size & inflation :doh
GPS is the only way :thumbs
My Mav's got standard wheels and tyres, got roof bars across the top, filled up today it worked out to 32 mpg. Mind you I've driven like my grandma on the last tank.
illy
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