Drag from roof box vs trailer

Submitted: Monday, Apr 23, 2007 at 13:53
ThreadID: 44633 Views:10051 Replies:8 FollowUps:5
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Does anyone have any hard data on whether fuel efficiency/general performance is affected by carrying a roof box vs towing a trailer? I'd like to avoid the hassle of a trailer if I can but I've heard it argued that a loaded trailer still uses less extra fuel than a roof luggage box. I'm in a Forester so doing all road driving rather than serious off road.

Any help/advice is appreciated.

Cheers, Muzz
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Reply By: Peter 2 - Monday, Apr 23, 2007 at 14:49

Monday, Apr 23, 2007 at 14:49
Having used both a roofrack and bag and/or towing a Cub off road supamatic behind a naturally aspirated diesel troopy for nearly 25 years they both create the same increase in fuel consumption which in our case was about 2mpg for one or the other or if both were used at once about 3mpg, the latter probably due to the icreased weight as much as anything.
This was cruising in the 90 -95 kph range, past that speed the words troopy and economy don't equate.
AnswerID: 235308

Follow Up By: Member - Oldplodder (QLD) - Monday, Apr 23, 2007 at 17:01

Monday, Apr 23, 2007 at 17:01
Found the same thing. About the same speeds.
Roof rack packed about 400mm high from the roof. Used it for about 15 years touring.
Now have CT which ways about 800kg.
The gear which used to go on the roof rack now goes in the CT.
(OK, 100kg on the roof rack, plus 100kg from the back of the car to the CT, plus another 100kg in extra gear now we have the room :o))

About the same increase in fuel costs, but the total mass being pushed up the hills is now 500kg more (bare trailer), so fuel economy drops lower if hilly with the CT.
FollowupID: 496410

Reply By: Granpa Joe - Monday, Apr 23, 2007 at 16:09

Monday, Apr 23, 2007 at 16:09
It would depend more on the size of the roof top box as an eg:-I Used to take a tinny on the roof of my old turbo deisel 2.8 pajero up to Urunga and back from Sydney on occasions and found it to be cheaper whacking it upside down on a box trailer behind the 4WD as there was less constant drag on the vehicle. Doing 110kmh most of the freeway, it made a noticable saving by having it on the trailer instead of the roof.
Reducing the aerodynamic drag (footprint) at constant highway speeds will make a difference although it has to be a substantial amount to warrant a trailer especially if the trip envolves more hilly terrain and a petrol motor.
AnswerID: 235325

Reply By: Member - Vincent A M (NSW) - Monday, Apr 23, 2007 at 18:51

Monday, Apr 23, 2007 at 18:51
All I can say is I use more fuel if the boat is on the roof .( I build boats on the south coast of NSW & deliver the same craft to Sydney (have for 15 years) I travel the same way, same type of traffic,.. & use more fuel & one gear lower going up Mt Owsley than if I tow the same craft on a 350 kg trailer, the windage is the problem. I think you will find the forester to be worse my wife had the turbo wagon & it hated anything on the roof. good luck
AnswerID: 235361

Reply By: Member - Roachie (SA) - Monday, Apr 23, 2007 at 20:09

Monday, Apr 23, 2007 at 20:09
In my experience, the answer depends largely on what type of terrain you will be travelling through and at what speed.

If you were driving up and down the Snowy Mountains highway at legal speed limit, the weight of the trailer would add significantly to your fuel consumption. Even if you slow down to say 70 k/h, the weight factor is still gunna hurt.

However, if you had a roof rack loaded up, the wind drag factor at slower speed will have less effect.

However, say you were crossing the Hay Plain (or Nullabor etc), then a properly loaded trailer will create less frontal area than a roof rack. At legal speed limit, there will be less impact on fuel economy from towing a trailer, than there would be from trying to jockey a block-o-flats which is 8 foot high.
AnswerID: 235396

Follow Up By: Granpa Joe - Monday, Apr 23, 2007 at 20:17

Monday, Apr 23, 2007 at 20:17
Well said Roachie, That's exactly the point I was trying to explain.... depends if your climbing hills or rolling along on the flat.
FollowupID: 496490

Reply By: AdrianLR (VIC) - Monday, Apr 23, 2007 at 20:21

Monday, Apr 23, 2007 at 20:21
We've had all three versions - roof rack, roof box and camper trailer. At highway speeds the roof rack with an irregular load was very noticeable (both in terms of performance and economy). The camper (around 650kg loaded) is only noticeable in hills but on the highway only marignally. The Thule roof box has no effect on performance or economy.

You need to position the box so that the curve of the windscreen continues into the front of the box or if this is not possible then behind the line of the windscreen. Vehicles were 2.8GU, Disco Td5 and Pajero DiD. Only had the full rack on the Patrol though.

AnswerID: 235401

Reply By: muzz63 - Tuesday, Apr 24, 2007 at 00:03

Tuesday, Apr 24, 2007 at 00:03
Thanks heaps for all your replies - I'll be able to argue the case for a roof box with the missus now. My main thought is I can confine her to about 65kg on the roof but once the trailer is hooked on she'll be wanting to put about 650kg on board.

Thanks again for all your replies - this information is gold.

Cheers, Muzz
AnswerID: 235459

Follow Up By: Anja from Top End Explorer Tours - Tuesday, Apr 24, 2007 at 00:16

Tuesday, Apr 24, 2007 at 00:16
On our last trip, we took the boxes of the roof and put them on top of the trailer.

Cheers Steve
FollowupID: 496590

Reply By: Dean - Tuesday, Apr 24, 2007 at 09:53

Tuesday, Apr 24, 2007 at 09:53
Sometimes I use the roof basket with a canvas bag, sometimes nothing on top, whatever the mileage is about the same.
AnswerID: 235492

Reply By: Russ n Sue - Tuesday, Apr 24, 2007 at 10:30

Tuesday, Apr 24, 2007 at 10:30
Jesus! We're buggered! In four weeks we take off with a Kimberley Kamper loaded to the gills and a boat on top, as well as a roof rack on the car with four Space Cases on it.

Sounds like we won't be driving past too many servos - even with the long range tank!


AnswerID: 235500

Follow Up By: Member - Stephen M (NSW) - Tuesday, Apr 24, 2007 at 13:00

Tuesday, Apr 24, 2007 at 13:00
Hi there Russ, was just having a look at your rig pics. How do you get away with where the rear spare is located obstructing you drivers side rear lights ?? Just curious thats all , or is the spare not normally in that position just looks that way in the photo's ??. Regards steve M
FollowupID: 496656

Follow Up By: SA_Patrol - Tuesday, Apr 24, 2007 at 14:58

Tuesday, Apr 24, 2007 at 14:58
All Kaymar bars have some kind of lights at the bottom, looks like he has the LED strips you can just see the LH side.
FollowupID: 496681

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