roof-rack vs fuel economy

Submitted: Friday, Jun 15, 2007 at 11:29
ThreadID: 46693 Views:2419 Replies:10 FollowUps:4
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does anyone have a rough percentile on how much extra fuel a roof-rack will use - at variying speeds...

my best economy is around 72k/h....

IYHO, do you think by removing the racks i will get it to 85k/h??

at 85k/h i can speed up economically enough not to be a bother, but not from 72k/h

wind resitence is expotential, that much i know... but if the darn rr's are adding 20% to fuel cost at 95k/h, they gota' go...
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Reply By: Member - TonyG (Qld) - Friday, Jun 15, 2007 at 11:34

Friday, Jun 15, 2007 at 11:34
Hi Rbt,

I do not have exact figures, but I was told that roof racks increase drag on the vehicle, which adds approx 10-15% fuel usage.

Also, the higher the speed, the increase in drag on the vehicle

AnswerID: 247018

Reply By: Redback - Friday, Jun 15, 2007 at 11:37

Friday, Jun 15, 2007 at 11:37
I dunno about the roofracks fuel economy but your car will use more fuel with a roofrack on :))

Between 5% to 10% more fuel

AnswerID: 247021

Reply By: Mark & Jo, S/side, Bris - Friday, Jun 15, 2007 at 11:38

Friday, Jun 15, 2007 at 11:38
Gotta say I have never really paid attention to any extra fuel usage from having the rack on, but there would certainly have to be some difference because I feel the difference of no roof rack to driving with roof rack. No so much with 100 odd km driving but more with local and take off.

AnswerID: 247022

Reply By: traveller2 - Friday, Jun 15, 2007 at 11:53

Friday, Jun 15, 2007 at 11:53
My troopy used to use approx 2mpg extra with the roofrack on when travelling at around 95 -100 kph.
Towing the 500kg camper used about the same, with both the roofrack and the camper about 3mpg extra.
AnswerID: 247027

Reply By: Member - Oldplodder (QLD) - Friday, Jun 15, 2007 at 12:06

Friday, Jun 15, 2007 at 12:06
Your best economy will occur at around maximum torque in top gear.
For my pajero, max torque is about 2000 rpm, so 75km/hr is best for economy.
(1996 2.8l diesel with 200,000 plus K, bull bar, 32" tyres)
Friend with 3.5l V6 petrol and similar car has best at about 85/90km/hr.

With car full and roof rack empty and roof rack full have noted the following figures.
Approximate figures on flat terrain and with reasonably steady throttle.
These figures are what I use to judge distance between fuel stops.

speed none empty full (300mm high)

80 9 9.5 10

90 9.5 10.5 11

100 10.5 11-12 13+

110 12 13-14 15-16

Wife driving, add another 1l/100k.
AnswerID: 247030

Follow Up By: Member - Duncs - Friday, Jun 15, 2007 at 12:31

Friday, Jun 15, 2007 at 12:31
You could get into trouble for the last line.

Even if it is true.

Sorry...............that should be more trouble.


FollowupID: 507857

Follow Up By: rbt - Friday, Jun 15, 2007 at 13:09

Friday, Jun 15, 2007 at 13:09
many thanks for this reply plodder...

i have a 2.0L 4g63 sohc naturally-aspirated & snorkled SWB Pajero with a van grafted on, ala '85 L300 4x4

now b4 you'all start smirkin I've had Rangys, F20's, Bikes an a couple of FJ55's in the past an i lurve this cheap van...

to his immense credit the bloke b4 me went to town on it with a quality customised interior, lift, wheels, snork, rhinos, pipe, motor etc..etc and chekr-plate detailing everwhere.. i even like the 2.o liter after reading some history on it...

with 15' 31R's it makes a great short-haul tuff-truck but on the highways i am at war with the truckies, so i try to take alternative routes instead...

however - economy and wind resitence and fuel quality is on the mind.... then the bloke camped next door make an offer for the RR's and im thinkin about it...

Despite keeping the load to the height of the basket and aerodynamically shaped I am sure i can feel it too much

One thing losing the rr's will take away is the "tuff-look" ... this is proving a bigger struggle than i expected ..

FollowupID: 507865

Follow Up By: Member - Oldplodder (QLD) - Friday, Jun 15, 2007 at 13:33

Friday, Jun 15, 2007 at 13:33

Used to have a 2.6l 85 NB long wheel base, with the same full length roof rack (2.1m). The 2.6 was rated at 74kw, while the 2.8 diesel is 92kw.
Economy was a little worse, but maybe only by 0.5l/100k.

One thing I found with mine is the position of the roof rack.

If I keep it back so the front of the rack is behind the rake of the windscreen, it saves about 1l/100k. Must be some extra resistance with the wind coming off the screen and hitting the underside of the rack.

Noticed I was getting more insects there on one trip.
FollowupID: 507867

Follow Up By: Member - Oldplodder (QLD) - Friday, Jun 15, 2007 at 13:39

Friday, Jun 15, 2007 at 13:39

No problem :o)

Wife knows, and it actually may be 2l/100k sometimes.

But then she always gets there before I would. :o)
Just has a heavier foot.

If we are in a hurry, she drives.
FollowupID: 507868

Reply By: Robin Miller - Friday, Jun 15, 2007 at 14:37

Friday, Jun 15, 2007 at 14:37
Wind resistance isn't dominant factor until around 90k so while a roof rack lowers your enconomy by a small amount (<5%) it will not shift you most efficent speed very much as its already below when its a major factor.

However I prefer Old Plodders statement that best efficentcy is at max torque - now I can justify sitting on 143kmh

Robin Miller
Robin Miller

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AnswerID: 247048

Reply By: jeffwa - Friday, Jun 15, 2007 at 15:03

Friday, Jun 15, 2007 at 15:03
Mines only a 3/4 length rack and having it sit back further allows the wind off the winscreen to bypass it mostly with normaly driving creatng minimal extra fuel useage. When it's loaded up with crap, it's another story.
AnswerID: 247053

Reply By: rbt - Friday, Jun 15, 2007 at 15:40

Friday, Jun 15, 2007 at 15:40
my engine is prolly too worn to sit on the higher edge of the torqu curve but it does sit on the lower edge of it very well...

mines a 3/4 too and placed to the back...sometimes i think too far behind the rake of the screen..

and thinking about it the cheap blue sheet i have spreaded over it hasnt destraoyed itself like previoous vehicles... in fact, this sheet has done nigh on 3000 klms so it mustent be doing too bad..

ok, hmmm, accept the lower torque curve, take the side roads and be happy - i am living it after all..
AnswerID: 247061

Reply By: Brew34.5(SA) - Friday, Jun 15, 2007 at 16:57

Friday, Jun 15, 2007 at 16:57
Bout 10 % diff to my patrol.
AnswerID: 247079

Reply By: Member - Duncan W (WA) - Friday, Jun 15, 2007 at 18:41

Friday, Jun 15, 2007 at 18:41
would say about 10-15% but I've given up worrying. I just drive to the road conditions and limit. No doubt costs me more but what the heck.
Make sure you give back more than you take

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AnswerID: 247114

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