Roof ski boxes, wind drag and mileage

[Mendieta]

Pretty much when I started skiing 3 years ago, I got a ski roof-box for the car (aka Pugmobile). It made sense at the time because we would normally fit four people in the car, plus the skis, baggage, etc. However, for the last couple years, it went unused because it's mostly two of us heading there, or just solo day trips, or we just take two cars because of logistics in long trips. So, I decided to remove it, since I didn't like the road noise and the unnecessary bulk.

Wow. I only took one trip since then, 3.5 hours to Tahoe. On the way there, I kind of noticed the difference in mileage but it wasn't too focused on that. On the way back, I filled the tank right before heading home, and I got home with a little less than half a tank. This is considerably more highway mileage than I used to get with the box mounted on the roof.

I searched around in the "Internets", and I found all sort of data. Data are anywhere in the 3% to 40% mileage penalty when using a roof box. I think a wide range of results is expected since because there are so many variables in play. Incidentally, car design involves a lot of thinking about minimizing drag (aerodynamics), as we all know. This article scratches the tip of the iceberg, and also discusses some of the fundamentals of air drag (See also here).

Anyway, I wonder if any of you have any hard (or soft) data from your own experience with using roof-box (or roof-rack) on mileage. Something like highway mileage, with and without the box. We could compile some numbers. Neeeerdy, yayy! Before anybody asks, sorry, but no, I don't have the data and I am not spending 2 hours mounting my box again for a test. And yes, I bought a cheaper box that takes a degree in mechanical engineering to put together.

Thanks!

 

[crgildart]

09 Forester 2,5.. Average without Thule 21 (skinny and tall with a spoiler on the back) was around 21.5. Average with the box between 20,5 and 21 depending on the wind direction..

 

[Mendieta]

09 Forester 2,5.. Average without Thule 21 (skinny and tall with a spoiler on the back) was around 21.5. Average with the box between 20,5 and 21 depending on the wind direction..

Yeah, that's one of the factors, my box was really tall, on the other side of the spectrum from skinny. Thanks for the data!

 

[Chris Walker]

I take my box off for the summer, so it's on half the time and off half the time. Logically there has to be an effect on mileage, but I can't detect it.

 

[crgildart]

There are some who claim to get BETTER mileage with the ski box. Perhaps it can act as a spoiler to disrupt the low pressure suck on the tail of the vehicle causing more positives behind than the negatives on the front?

 

[Doug Briggs]

I haven't checked with or without. I'm guessing there is a negligible difference.

I'd forgotten what my truck looks like when clean.

 

[scott43]

I'm always cautious when taking people's mileage numbers... There are many many conditions that affect mileage. Temperature, air pressure, altitude, speed, octane. It's not that repeatable. Having said that, I'm sure it affects mileage, more the faster you go. I was looking at a trailer hitch box instead..you can't put skis in it..but it's useful for everything else..and it's out of the air flow.

 

[Mendieta]

Having said that, I'm sure it affects mileage, more the faster you go

Yes, the dependency is (force) roughly proportional to the square of speed. I am pretty sure there is some break even point under which speed (and therefore drag) doesn't matter. This is also why I was suggesting highway mileage. Cheers!

 

[cantunamunch]

I'm always cautious when taking people's mileage numbers... There are many many conditions that affect mileage. Temperature, air pressure, altitude, speed, octane. It's not that repeatable. Having said that, I'm sure it affects mileage, more the faster you go. I was looking at a trailer hitch box instead..you can't put skis in it..but it's useful for everything else..and it's out of the air flow.

That's why I do a three-tank-average mileage in a separate column.

Previous box vehicle (VW) was 24 (24.7 3TA in October 2017) with no box and 20 (20.8 3TA in January 2017) with the box. That 20.8 came with a LOT of windage correction - side gusts were nightmares and driving in a windstorm on wet snow was a white knuckle affair with the box on.

Haven't mounted the box on the Jeep yet.

 

[Plai]

2008 4Runner V6 w/ Yakima Skybox ... get around 19 mpg w/o skybox and 18 mpg w/. This is the average on trips up to the sierra and back, summer or winter (with winter numbers 1 mpg less given the winter formulation). Not enough for me to worry about.

 

[Mendieta]

2008 4Runner V6 w/ Yakima Skybox ... get around 19 mpg w/o skybox and 18 mpg w/. This is the average on trips up to the sierra and back, summer or winter (with winter numbers 1 mpg less given the winter formulation). Not enough for me to worry about.

Thanks! That's a pretty slim box, right?

 

[Plai]

Thanks! That's a pretty slim box, right?

I "want" to say "yes", but not certain ....

https://www.rei.com/product/876510/yakima-skybox-16-carbonite-cargo-roof-box

 

[Banzaibikes]

I noticed a 1-2 mpg dif on my 2006 CRV with roofbox.

 

[Mendieta]

I noticed a 1-2 mpg dif on my 2006 CRV with roofbox.

Thank you! I'll add it to the stats, what's your typical highway MPG?

 

[John Webb]

I haven't checked with or without. I'm guessing there is a negligible difference.

View attachment 36811

I'd forgotten what my truck looks like when clean.

With that setup your MPG's may be better WITH the box. It's mostly out of airflow and smooths out drag/turbulence in the truck bed.

 

[Banzaibikes]

Thank you! I'll add it to the stats, what's your typical highway MPG?

Hi way, normal speeds I get 25-27 tops. I average 21-22mpg city/hiway.

 

[JeffB]

2016 MB GLC 300 with Thule Pulse M on 48” Aero bars.

Bars only: 28.7mpg over 720 miles at 68.4mph avg speed. (Car computer calculated numbers).

With box: 24.3mpg, same route and distance return trip, 63.1mph avg speed. (Car calculated). Also was +200 pounds or thereabouts in cargo.

Summer temps in the southeast, 93 octane, between sea level and 1k feet.

I kept track because so many variables were the same and it seemed like an interesting thing to do on a long solo road trip. And to see if my lovely bride would roll her eyes or call me a nerd when I shared the results with her. Answer: both.

 

[John O]

2008 Subaru Outback, without the box on I can get about 27-28mpg highway. With the box on, it's more like 25-26mpg. This was with a yakima skybox 16. This year I got a new thule motion xt and haven't paid attention to the mileage yet. They've very similar shapes so I'm expecting it to be about the same.

 

[LegacyGT]

I've got a 2015 Dodge Durango V6 with a Yakima Skybox 16. I previously had the same box mounted on a 2009 Mazda CX-9 and a 2005 Subaru Legacy GT Wagon. My experience with the three cars is pretty much the same. Around town there's really no difference. On the highway around the speed limit there starts to be a small penalty...maybe about 10%. At higher speed the penalty increases but at 70-80 mpg it still probably isn't as much as 20%. (Remember that higher speeds hurt your mpg even without a roof box.) Some boxes are pretty aerodynamic and they may interact differently with different car body styles. Still, whatever the drag coefficient, there's no getting away from the additional frontal area. It's gonna hurt to some degree no matter what.

As others have mentioned, driving style and other conditions will impact your mpg more than anything. All things being equal, the roof box is going to hurt you a bit, but more noticeably at higher speeds.

 

[Posaune]

I take my box off for the summer, so it's on half the time and off half the time. Logically there has to be an effect on mileage, but I can't detect it.

This is my experience.

Subaru Imprezza, Thule "Forester."