Roofbox effect on MPG

[Deleted User]

Going on a long journey today. 4 hrs non-motorway driving.

Was going to take the roofbox off to save MPG but might not bother after reading this. (Its fiddly to take off and on).

http://www.autoexpress.co.uk/news/autoexpressnews/229776/the_mpg_mythbusters.html

Hammyman

Hang on...

It says that the well known facts about open windows etc is wrong and then in every example, goes on to record DROPS in MPG?

They started with a baseline figure of 30.9MPG.

They noticed a 8% drop in economy with aircon on at typical engine speeds but none when on an artifical run so its bunkum?

They dropped 0.3 MPG with the windows open so its bunkum?

They dropped 0.8MPG with low tyre pressure so its bunkum?

They dropped 0.2MPG with the roofbox so its bunkum?

So they started off at 30.9MPG but after 24.1mpg – Roof bars and laden roofbox, plus three adults in the rear, a bike carrier with two adult bikes and air-con it went down to 24.1 MPG but apparently its all rubbish?

The only thing that's rubbish is the article. I guess it was a slow news day.

I like this comment:
- Slowing down on the motorway will boost fuel economy dramatically.

Guess why? You're decreasing wind resistance. What does a roofbox do? Increase it. For the love of god....

ailuro2

iirc a roofbox eats petrol / diesel at a rate of around 10% extra, aircon is 5% extra, as is an open window.

If you're not using it take it off, and keep it off till you need it next.;)

forgotmyname

Made a big difference to my old 2L petrol mondeo hatchback, Going to Bude done the trip several years in a row,
Normally got 1/4 of a tank left, With the roofbox i was close to being in the red.

Open windows around town wont make much difference where aircon will, Thats common sense. The car has to increase
the revs or amount of fuel being used to compensate for the extra load. Where at higher speeds the engine is producing
enough power to not require any extra fuel.

MX5huggy

Driven to the alps twice in the same X3 once in winter so no aircon but with a roof box (full of skis etc) & 4 adults 22mpg, summer so air con on but no roof box, 2 adults 26mpg. It a 3l petrol BTW.

dannymccann

RE: Aircon, I always use my rule of thumb - under 30mph windows down no aircon, over 30mph windows up aircon on. Benefit to other organs as well, such as my ears and the ability to talk to your passengers, try doing that at 70mph going down the motorway with trucks and cars either side with windows all down