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Roof box gas
- Thread starter daught
- Start date
I've been researching as well because I want to have one installed. ive read to expect a minimum of 15-20% reduction in mileage with it on. Mind you ill only put it on when needed and not regular use.
daught
Well-known member
Thing is I have about 7000KM of driving ahead of me. I dont need the trunk space for most of those. But once I get out west having a free trunk in "RV" would make my life way easier. No packing and moving stuff at night and in the morning. I can just keep organized in the top box. I wish I could find a top box that fit in my trunk(vagon), but most are way longer.
I find that hard to believe
I've seen roof bags. Same sort of idea but they are a heavy rubber/canvas/something type of material. Easy to collapse down when not in use.
oh?
There's a ton of info out there and I'm sure a lot of it is BS...
In the end it depends on the car and how you drive a lot as well as what type of top box you have. Smaller engine with a box on top will lose more efficiency than a car with a a larger engine. More work required to move through the air.
As for efficiency loss...I've read anywhere from 5% for some, to 50% for others in the amount of extra fuel being burned. I've got a Yakima Scout box and just trying to find some cross bars as my car doesn't have factory roof rails...unfortunately Canada is the ONLY place in the world that my car doesn't have an option for the roof rack.
Assuming you don't drive a car with a slippery shape that the roof box messes up, it should be proportional to the change in frontal area of the car. For example, a Golf MKVII is around 2.2 m^2 frontal area, a yakima skybox 21 (long slipperly looking box) is ~0.41 m^2 so I would expect ~20% reduction in fuel economy if it didn't mess up the cd of the car (which it probably would). The giant canadian tire beige cube would be much worse.
EDIT:
The above calculation assumes you are moving at a reasonable speed and wind resistance is the primary driver of fuel consumption. If you are driving around town, you may not notice much difference at all.
EDIT:
The above calculation assumes you are moving at a reasonable speed and wind resistance is the primary driver of fuel consumption. If you are driving around town, you may not notice much difference at all.
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I used to use a Yakima space case, the big one that would hold 215cm skiis. Or a body. There was definite wind resistance and noticeable decline in mileage but I cant put a number on it. On the mini van it was less obvious, on my jetta it stuck out a ft past the windshield and caught breeze. It cost about $550. but parts are available and it was well worth the money. I would never put a soft bag on my car roof.
if you're not pulling a trailer there's mini trailers about the size of the roof box, should have minimal effect on mileage.
The removable roof rack (just the rack, no box, etc.) I have for my car results in 3% worse mileage on the highway. It stays in my shed unless needed for this reason. It is straight square bars and not very aero, the car itself has a very low CD. Adds lots of wind noise as well.
So I would not be surprised at some of the numbers here.
Best solution would be to find one that fits in the back for the part of the trip you do not need it. Next would be to order one for pickup at the star of the leg you will need it--a little risk here that it is not there when you get there.
So I would not be surprised at some of the numbers here.
Best solution would be to find one that fits in the back for the part of the trip you do not need it. Next would be to order one for pickup at the star of the leg you will need it--a little risk here that it is not there when you get there.
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It's quite possible that something of this sort could increase fuel consumption by 20% and it wouldn't surprise me if it were more than that in highway driving.
Modern cars are highly optimized for aerodynamics, and the flow over the roof is a critical area. The roof box increases the frontal area and as a double whammy, it increases the drag coefficient. A small trailer, or a box cantilevered on a trailer hitch, so that it is all within the car's frontal area, will likely do better.
My bike hauler is a great big box (Ram ProMaster a.k.a. Fiat Ducato full-size van) and believe it or not, Cd = 0.31 for that vehicle. Contractors who put ladders on the outside of the van say they absolutely destroy the fuel consumption ... 20% - 25% worse in highway driving just from having a ladder on the roof.
Modern cars are highly optimized for aerodynamics, and the flow over the roof is a critical area. The roof box increases the frontal area and as a double whammy, it increases the drag coefficient. A small trailer, or a box cantilevered on a trailer hitch, so that it is all within the car's frontal area, will likely do better.
My bike hauler is a great big box (Ram ProMaster a.k.a. Fiat Ducato full-size van) and believe it or not, Cd = 0.31 for that vehicle. Contractors who put ladders on the outside of the van say they absolutely destroy the fuel consumption ... 20% - 25% worse in highway driving just from having a ladder on the roof.
How about a hitch mounted cargo shelf? If you're not trying to parallel park in a dense city that seems like the best option, to me. Should barely affect mileage at all.
I had the same problem with my car. The ONLY place I could find them was at Rhinorack http://www.rhinorack.ca/
They are expensive but if you switch cars all you need to get for the new one is the Fitting Kit
Which car?
Dave L
Well-known member
The soft bags also flutter in the wind really bad and as a result are really noisy. I had one and got rid of it after a single trip.
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