Drag from roof box vs trailer

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.

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.

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

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.

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.

Adrian

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

Sometimes I use the roof basket with a canvas bag, sometimes nothing on top, whatever the mileage is about the same.
Dean

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!

Cheers,

Russ.