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How much does a roof rack actually affect fuel economy?
by
JBG3
on 05 Mar, 2012 13:05
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Looking for anybody's experience/numbers on this. Im loving my mk2 now that its actually running, but of late, ive become obsessed with its fuel economy.
On all my DDs ive had, I always run a roof rack. Ive had a Yakima brand one for years, and just today moved over the gutter clamps, cross bars, and fairing to my 86 jetta. I leave it up there year round too, ladders and material for work, skiing in the winter, biking, boating the rest of the year. Not a statement thing, but a utility thing, just a PITA to remove all the time, so it stays forever.
Im curious what I can expect to lose in fuel economy after installing the rack typically. I was getting 40-44 mpg before, and with 4 snow tires on it. Im hoping the fact that I added the fairing will preserve some economy for me, but the towers, cross bars and fairing are big for this car, so there is a lot of extra bar on the outside, and the fairing is at a pretty steep angle.
Would be interesting to know what other people have encountered.
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#1
by
Jetmugg
on 05 Mar, 2012 13:45
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I can't put a hard number on it, but having the rack on definitely makes a difference. I have a Mercedes 190 Diesel that normally sports a Yakima rack with a fairing, 2 bike trays, and a couple of spare wheel mounts (bicycle wheels). With the fairing and 50" bars, you are increasing the frontal area of the car by more than a foot. This increase in frontal area is probably on the order of 4-5% of the total frontal area of the car, and the coefficient of drag is probably affected even more.
If your experience is like mine, you can hear a lot of wind noise associated with having the rack on versus not having the rack on.
My "guesstimate" is that the roof rack costs somewhere in the range of 4-5% of the total fuel economy. At your 44 mpg number, my opinion is that you might pick up an extra 2mpg at highway speed if you lose the rack.
SteveM.
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#2
by
mystery3
on 05 Mar, 2012 22:26
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I'd guesstimate the same 1-3mpg. I frequently ran a thule rack with fairing on my old e30 and the difference I calculated was usually in the 1.5mpg region. I had also cut the bars on that vehicle down to the bare minimum because I hate the way they look with overhang.
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#3
by
ORCoaster
on 07 Mar, 2012 19:08
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Doing a quick search I see valid numbers ranging in the 5 -10% area. Best solution of all. Leave the rack on and slow down and adjust your driving habits to gradual accel, slow decel and plan your route to save miles driven. Doing that can reduce your driving stress and you still won't be late to appointments. Best of all you might be able to keep the rack AND get better mileage.
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#4
by
Mark(The Miser)UK
on 08 Mar, 2012 02:28
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Get a Quantum roof rack, the best ever made by Audl/VW Aerodynamic and easy to remove bars when not needed.
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#5
by
JBG3
on 12 Mar, 2012 05:43
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thanks for all the replies everybody!
I have some tentative numbers now, that might be thrown off because half my tank was around town driving, but I dropped from 44mpg to 38mpg! The 44mpg was all highway driving, so I intend to leave the rack on there until I duplicate that same run of all highway driving so I can get an exact comparison.
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#6
by
JBG3
on 12 Mar, 2012 05:56
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I'd guesstimate the same 1-3mpg. I frequently ran a thule rack with fairing on my old e30 and the difference I calculated was usually in the 1.5mpg region. I had also cut the bars on that vehicle down to the bare minimum because I hate the way they look with overhang.
you would hate the way this looks then. I have to say, its a little to wide even for me, I keep catching my head on it when exiting. The bars and towers were originally bought for a MB 123 diesel, which is a bit wider, and has a lower and tougher gutter, so the rack is not quite as high. These are 58" bars, and hang past the towers almost 10 inches on both sides. Also because of the difference in height, the fairing is far less sloped than it was on the MB. Strangely, it doesn't make any noise at all. Not that I can tell anyway
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#7
by
JBG3
on 12 Mar, 2012 05:59
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Get a Quantum roof rack, the best ever made by Audl/VW Aerodynamic and easy to remove bars when not needed.
Those are really thin on the ground over here, BUT, I do happen to own a quantum wagon GL-5, and you are right, its way sleeker and those bars are super easy to remove, even has a pouch for you to store them in the trunk. Ill have to keep my eyes open, the car in the pic here is being potentially sold, and I can't be stealing parts off it after the guy took a look at it!
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#8
by
mystery3
on 12 Mar, 2012 22:31
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I'd guesstimate the same 1-3mpg. I frequently ran a thule rack with fairing on my old e30 and the difference I calculated was usually in the 1.5mpg region. I had also cut the bars on that vehicle down to the bare minimum because I hate the way they look with overhang.
you would hate the way this looks then. I have to say, its a little to wide even for me, I keep catching my head on it when exiting. The bars and towers were originally bought for a MB 123 diesel, which is a bit wider, and has a lower and tougher gutter, so the rack is not quite as high. These are 58" bars, and hang past the towers almost 10 inches on both sides. Also because of the difference in height, the fairing is far less sloped than it was on the MB. Strangely, it doesn't make any noise at all. Not that I can tell anyway
In California you'd have to run red flags on those bars. I think the unmarked overhang limit is 18"
Why don't you chop 'em off?
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#9
by
JBG3
on 13 Mar, 2012 08:23
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In California you'd have to run red flags on those bars. I think the unmarked overhang limit is 18"
Why don't you chop 'em off?
Cause those dam bars cost me 70 bucks.
Actually, im thinking of moving the whole setup over to another car thats wider. I don't really like how the generic gutter clamps grab the rubber/plastic? roof rail on this car. Not as strong as id like it. Might get some specific towers for this car that will work better.
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#10
by
Mark(The Miser)UK
on 13 Mar, 2012 10:23
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Get a Quantum roof rack, the best ever made by Audl/VW Aerodynamic and easy to remove bars when not needed.
Those are really thin on the ground over here, BUT, I do happen to own a quantum wagon GL-5, and you are right, its way sleeker and those bars are super easy to remove, even has a pouch for you to store them in the trunk. Ill have to keep my eyes open, the car in the pic here is being potentially sold, and I can't be stealing parts off it after the guy took a look at it!
Easy to swap to a 1.6TD.
Although the best best roof rack it is a mystery as to why they didn't level off the feet, so that the 4 rungs are level.
Another set of holes in the top rail would be nice, and possibly extending the rail to the front pillars, unless there is an issue of cracking the windscreen.
Even as is; another 2 rails nearer the ends of the rails would hel carrying the longer stuff. I've carried over 250kg on those rails and 18ft long items...
I have two wagons and four sets of cross rails to play with. To think in my youth, I actually scrapped a set 
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#11
by
JBG3
on 14 Mar, 2012 10:30
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wow, I can't break 40mpg with this yakima rack! best I can do is 39 something. All conditions are the same, and the same drive at the same speed that yielded me 44mpg with a bare roof will only yield 39 with the rack. 5 mpg less is unacceptable!
Ill move it off, and try something else. The Quantum rack in the pic above is not for grabs by me, ill take a look in the yards for some efficient looking racks that might fit on this roof
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#12
by
Mark(The Miser)UK
on 15 Mar, 2012 03:46
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wow, I can't break 40mpg with this yakima rack! best I can do is 39 something. All conditions are the same, and the same drive at the same speed that yielded me 44mpg with a bare roof will only yield 39 with the rack. 5 mpg less is unacceptable!
Ill move it off, and try something else. The Quantum rack in the pic above is not for grabs by me, ill take a look in the yards for some efficient looking racks that might fit on this roof
Sell those bars to a gymnast who likes parallel bars...
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#13
by
JBG3
on 15 Mar, 2012 10:04
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Sell those bars to a gymnast who likes parallel bars...
lol, Im going to put it on the GF's car, should be just about perfect. Shes got the same model car I originally bought this for, so it will be an exact match
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#14
by
ORCoaster
on 15 Mar, 2012 11:16
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Great, spread the MPG reduction with a loved one. Da heck?