Does it affect mpg?

[Deleted account DD]

I usually leave my roof bars (not the rails that run the length of the car theyre bolted on i mean the bars that go cross ways) on all the time, its handy and Im too lazy to take them off

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

[zippy656]

nope. but it MUST cost you something..

maybe 10 miles a tank full would be my GUESS>.

[Adz]

It will cause drag & noise maybe 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

[zippy656]

oooo tosh.. not used that for ages!!

what about the roof lights... that make any diference?

[extreme-4x4]

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]

ah right, thats why my truck sits better at 55 / 60 than 150 mph then.. right??



still thinking about my roof lights...

[extreme-4x4]

Originally Posted by Adz

It will cause drag & noise maybe 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

but you have bigger tyres on . so id expect better mpg probably balance it out and whats your top speed ?

[Adz]

If you put something solid like a roof box or tow a caravan for example then your causing drag

Putting lights on your roof the wind will go through / round the gaps

Are you telling me a roof ariel will cost you mpg THINK.!



Look at lorrys when your next on a major A road or Motorway

On a vehicle where every penny counts towards economical fuelling

If spot lights on there roofs (mine has them) affected mpg then they wouldn't put them there

[zippy656]

convinced me there mate...

[Adz]

Originally Posted by extreme-4x4

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...

[tezzer]

Originally Posted by extreme-4x4

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,

[BigBlack]

Does it affect MPG...... simple answer.... yes.....

Increased drag = increased fuel consumption... (FACT! ) 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

[extreme-4x4]

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]

Originally Posted by daved

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

NO
I do , surprisingly , have a life.

Oh and I'm too lazy to take mine off too

[(RIP) PLANK]

Originally Posted by Adz

Look at lorrys when your next on a major A road or Motorway

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.