Roofbox Efficienomics!
I bought a roof box many years ago to facilitate moving house and between countries. When you're moving a lot of stuff long distances roof boxes are a no brainer.
I've since taken it off as intuitively it's going be less fuel efficient using said box than without it, giving the increased drag...
I never really worried about calculating the relative fuel efficiencies, but now I've hit a life-phase where it would sort of be useful to have it some of the time, and taking it on and off is a bit of a hassle, so I got to wondering what the cost would be of leaving it on all the time!
It's actually not as straightforward as you might think finding data on this, given that roofboxes are so popular.
The most annoying thing is that the number one search return for 'roofbox fuel efficiency' returns an article by a roofbox company which lies, informing us that boxes don't have much of affect, without giving any figures!
Roofbox efficiency
However the most accurate article I found, based on an actual test of two common cars with and without boxes reveals that a certain brand of Dacia managed 54.2mpg without a roof box but only 48.5mpg with it.
That's a difference of 5.7 MPG or just over a 10% reduction.
At current diesel fuel prices that's £6.75 a gallon, so that's...
- £6.75 per 54.2 miles or 12.5 pence a mile compared to...
- £6.75 per 48.5 miles or 14 pence a mile.
At 1000 miles a month, my current average mileage (quite high I know!) that's an annual fuel bill difference of....
- £1250
- £1400.
Or £150 a year.
This difference MIGHT be bigger or smaller.....
Differences in fuel consumption between larger surface area objects vary considerably with speed travelled.
For example if you're driving at 40 MPH or below then there's minimal difference in fuel consumption.
However if you're driving at 70 MPH then the differences are larger, as large as 25% with a roof box compared to without one, according to some data sources!
That was based on an American comparison, long distant journeys, mostly on faster roads.
I'd guestimate that around 60% of my driving is motorway and faster A roads, so I think my difference is going to be nearer 12-15%.
So I think I need to work on £200 extra a year WITH the roofbox, YOWSER.
It's worth taking on and off when I don't use it, for sure!
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What about a Kombi?
Good suggest for a next vehicle, but that's a few years down the line!
May the bull market reward you sooner 😉!
If most of my driving were around town, I would leave it on. I'd only remove it for long trips.
One time, i removed the passenger seat, leaving only the driver seat and back bench. That gave me a ton of room.
Thinking outside the box there, excuse the pun!
Modern cards are pretty aerodynamic, so adding stuff on the roof is bound to have an effect. I've not used a roof box much, but then our main car (Honda CRV) is pretty roomy. It's only really when you have a full load of people and baggage that it gets tight. With our old Zafira you could take 7 people, but lost the boot space when full.
I think the most I've ever driven around is 4!
Yes indeed, made the same calculation once. And even the difference of taking off the box, but leaving the rack on is painful!
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