About how much will a Thule or Yakima roof box reduce mpg?

[mountainwalker]

I'll have a Thule roof box on our subie outback for the first time - I think it's about 16 liter size, big enough to handle 4 skis, boots, poles and equipment. It's aerodynamic. About how much will it reduce fuel efficiency?

 

I've heard that light haulers that attach to your trailer hitch are the most fuel efficient because they don't increase the aerodynamic profile of your car. However, they're quite expensive and seem less convenient.

[bulwnkl]

I'd be interested in the answer to this question as well, although my cost experience is backwards from the O.P.'s. I can buy a hitch, carrier rack for it, and some kind of tupperware box for less money than I can get just a Thule or Yakima roof-top box. Still, I'm interested in hearing what folks have experienced for mpg hit on the rooftops.

[four-fleet-feet]

I've had Yakima racks for 23 years now. Here's the scoop on mileage off the unloaded MPG as I've experienced it on 3 different vehicles.

 

Yakima raingutter 1A rack: 1 mile off. No change whether the airdam is on or not. Airdam's only on the front bar. Rear bar still has drag.

 

Upright bike mount: with bike = 2.5 off. Without, 2 off. I have an old upright Lockjaw and a touring Trek, with front low-rider racks and rear rack, no fenders. If you have a front skewer-clip design, that might be lower.

 

Basketcase, 2 off, empty, if you keep the net on it. Take the net off and it's 1.5 off. Fill it, and it depends on how HIGH your cargo is, and whether you have it in plastic or it's really sticking out everywhere. I have averaged 2 to 3 for tall loads depending on height of cargo.

 

Yakima RocketBox... here's the bad one. It's a big sail if you're headed downhill at speed. On most cars it overhangs the hood. (It's not recommended on the Imp for this reason.) In marginal weather it could surprise you when it lifts, especially if you have it on one side of the rack with nothing to balance it out. I could always depend on losing at least 2.5 MPG with it, but if there were nasty crosswinds, or really tough drops/ascents, I saw 4 off as I fought it all the way down bad passes. It was a tossup whether I wanted to pay more for gas or have less chance of losing my stuff. I still recommend it if you're hauling good gear. If you're hauling it alone on top, PUT IT IN THE MIDDLE. It may be harder to get into it, but you'll have better balance when the semis (or the $#@$&%$ winds) blast by you.

 

End result: I always ran the RocketBox (passenger-side opening only), and switched out between the Lockjaw or the BasketCase on the driver's side depending on where I was going. The vehicles seemed to handle easier if the Box wasn't on the roof alone. If I ran the Lockjaw I usually had a set of these little U-shaped brackets (the little oar/mast/you-name-it carriers, also good for 2x4's) on the outboard side of it.

 

However, if I didn't have an outrageous load in the Basket, or a taller/heavier bike (my vintage Trek 614 cro-mo weighs about 21 lbs and is a 49cm), I didn't go over 3 miles off my usual. So it's not cumulative (bike MPG+box MPG=OMG). The bike had drag but no sideways lift. The basket actually modified the sideways lift from winds when it was packed.

 

What can also eat mileage is a bug dam on your front hood - those eat a mile themselves. And low tires - don't get me started on low tires messing up your MPG.

 

Trailer hitch attachments might be more efficient, but they mess with your hatch access. You can trash your prized ride really easily, too. Cornering, backing up, tailgaters... I had one, before I went with Yakima. Go back to it? NEVER. Besides, they make getting into your hatch a real pain at rest areas or national parks (or anywhere else, for that matter). Even if they fold down, you'll probably end up unloading. The third time you do it you'll be putting ads up in every REI to unload it. I pity the person who bought mine!

 

My '91 RocketBox won't safely fit Dragon (too long for the Imp roofline). So I'm running the Basketcase and/or the Lockjaw now. It might be that I'm no longer attaching them to the Yakima factory bars, but I've noticed the basket's only hitting me for 1 mile off without the net. Yakima bars aren't as aerodynamic as the ones on a Subaru. Since the Subaru factory bars don't hold all the attachments, and some really do need the round crossbars (Yakima) or the Thule ones, your mileage may be better with attachment clips from Subaru on your Subaru factory roof rack instead of using adapters to hold the Yak/Thule factory bars on your roof IF your attachment can use them.

 

AFAIK, no enclosed box from either company can use them on the older (pre-98) non-OBW Subaru factory racks (certainly not the Imps or the OBS, although there IS a smaller Forester box, made by Yakima, Subaru-branded, which can be adapted to the '97-'99 Imp/OBS roof, but I have never seen a SOA bulletin stating it's safe). An established older dealership would still have the manufacturer's bulletin detailing rack/attachment safety. I believe the problem was the roof rack crossbar-attachment strength on the older Imp/Legacies. The newer ones should be able to, check Yakima or Thule's Fit Guides AND your nearest Subaru dealer if it was around when your car was new. (Brand-new dealerships don't have the older bulletins, BTDT)

 

Be advised the attachment clips from Subaru are a third the price of ones direct from Yakima - Dragon's 14H were $36 or so from Yakima, and only $13 from Puyallup Subaru. 4 to a set - the only difference is the Yakima brackets have new screws and nuts, and the Subaru ones are the brackets only. If you've lost your original mounting screws for the round Yakima or square Thule brackets which came with your attachments, it might be cheaper to get the Yak or Thule parts instead. You could always call them and ask how much the screws are!

[Spiffy]

I used this Yakima box (side opening) on our '98 imp... took it to California and back (about 1500 miles) with no problems...

 

had to move the mount holes back to get the box forward enough to open the rear hatch... used the stock mounts that came with it on the factory Subaru cross-bars... and we tried not to put anything too heavy in the very front...

 

also makes for a nice rain visor when stopped...

 

 

--Spiffy

[griffenrider]

I have the thule Ridgeline-4 bike rack (trailer hitch mount) and the Cascade XT (17 cf, I think) roof box. Running both at the same time I get about 26mpg on the highway (28 normally); doesn't seem to affect city driving at all, I get 22 either way. Running just the bike rack doesn't make a bit of difference, even on 1000 mile road trips.

[subsince77]

I have had a fairly large Barcrafters box on top of my 97 OBW. I used to take it off sometimes, but now I always leave it on there. I almost always have it half full of gear. I also had to move it forward so the hatch would open better. I only lost about 1 mpg on the highway. I still usually get between 28 and 31 depending on speed. A marked decrease when we are going over 70.

 

As far as bikes, we run a yakima fork mount roof rack on close to home trips. For longer trips I run a two bike hitch mount rack - huge difference in milage. Two bikes on top, 23 to 26 mpg, two bikes on the hitch mount 28 to 31 mpg.