Actual MPG Loss Roof Box

[gathermewool]

We’re going to drive 1200 miles down to visit my family as soon as we’re all vaccinated, hopefully around Thanksgiving.

We took the Legacy last time we drove down and averaged ~30 MPG, but want the extra storage provided by our Forester and Skybox 16 Carbonite (that is 3 years-old and has never been used ).

For reference we did take the Forester XT down to FL when it was relatively new and I believe we averaged 24 MPG without the roof box.

I‘d like to hear from everyone with a roof-top box. What hit, % wise, are you seeing in MPG?

Feel free to throw in general pre-checks, packing tips, etc to the mix. This thread can be a free for all, I don‘t care!

 

[skyactiv]

Can you not take the kitchen sink with you and pack light?

 

[E365]

I use a Thule Ascent 1100 all-winter, every-winter. I do a lot of freeway driving and it seems to drop my 70MPH MPG somewhere in the 4MPG - 6MPG range on my MINI Clubman. But, I’m not totally sure how much of that drop is just the general cold weather/winter tire/snow covered road MPG hit and what’s the box.

Maybe 15%-20% at 70MPH vs. summer conditions/setup (??)

 

[gathermewool]

Can you not take the kitchen sink with you and pack light?

That’s what I said! The answer from the wife is no. lol

On a serious note, even taking into consideration that we‘ll have access to a washer and dryer, can buy more diapers there, etc., we still end up with fully packed car with zero extra space. This time, we would like to have some room to bring luxuries, like the off-road-capable stroller for the to-be 3 year-old. It would be nice to have easy access to a larger cooler, too, instead of just our compact little one that we squish below my to-be 6 yo son’s feet.

I‘m sure I could come up with a few more examples, but they would be besides the point of the thread.

 

[Rand]

depends on speed. faster you go the worse it is.
i'd expect 3-5mpg.
my random guess:
if you would have gotten 25mpg@75mph?
maybe 21.

 

[gathermewool]

Also, and not to get too far off topic, we’re not 100% going to take the Forester and/or use the roof box. It’s an option. Compared to the cost of a flight + rental with car seats is well over a grand. Gas will be around $200 or less. We also don’t mind driving overnight to get there.

I’m pushing for using the Legacy, just like last time, because of the MUCH longer range and lower cost per trip (regular vs premium + MPG diff) - closer to $120.

 

[gathermewool]

depends on speed. faster you go the worse it is.
i'd expect 3-5mpg.
my random guess:
if you would have gotten 25mpg@75mph?
maybe 21.

Without the roof box you’re about spot on. Maybe closer to 24 MPG @ 75. We plan on doing no more than 5 over the entire way.

I’m also interested in driving dynamics. from calm to windy, what are you talking guys feeling?

Do you guys pack everything to be watertight or are these boxes pretty solidly water resistant?

 

[AZjeff]

Box on Legacy?

 

[Imp4]

Just a swag but I'd say 10%.

 

[Rand]

either have a trailer hitch?
one of those platforms about 50$ at HF and much less mpg penalty.

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[gathermewool]

Box on Legacy?

I haven‘t researches if that’s even possible. That would be ideal

 

[gathermewool]

Just a swag but I'd say 10%.

That was my guess. We may take a few hour trip before heading down to FL some time this summer. I’ll report back my findings for both that and the FL trip.

 

[gathermewool]

either have a trailer hitch?
one of those platforms about 50$ at HF and much less mpg penalty.

500 lb. Steel Cargo Carrier

Amazing deals on this 500Lb Deluxe Steel Cargo Carrier at Harbor Freight. Quality tools & low prices.

harborfreight.com

-20% off coupon

No hitch, but a good idea for the future. We already have the unused box. It’s got golf ball dimples, so it MUST be aero af!

 

[Cujet]

Just a thought, but it may be cheaper to send bulky but light stuff via UPS, ahead of time.

We do this when traveling and our little airplanes are full. Works out very well.

 

[BryanTDI12]

I have a Yakima Skybox 18cu. ft. model and used it on both a 2004 Audi Allroad and a 2016 Audi Etron with lots of highway miles. On the Etron, I saw at least a 40% decrease in mpg, which was horrible. I had the box slid all the way back as far as possible to eliminate overhang (still could see it if you looked directly under the windshield), but with three people, cargo box, 1.4turbo, and a good headwind, we swapped out for the other Audi. On the Allroad, at most I saw a 5% decrease doing 80mph. The box is suited much better for that longer car.

 

[Jetronic]

There's no speed limit advisory in the manual with a roof box fitted? Most manuals I've seen warn not to go faster than 100 kph (60 ish mph). The roof box manual has a similar warning usually.

 

[TacoFergie]

On my little 01 outback with a somewhat tired engine I noticed my interstate MPG (70-75mph) dropped about 4-5mpg unloaded. I know it's older and not in tip top shape (piston slap) but I even noticed the drag difference in drag when accelerating to interstate speeds.

 

[gathermewool]

Just a thought, but it may be cheaper to send bulky but light stuff via UPS, ahead of time.

We do this when traveling and our little airplanes are full. Works out very well.

Ive honestly never thought of that. We‘ll look into the cost.

 

[gathermewool]

I have a Yakima Skybox 18cu. ft. model and used it on both a 2004 Audi Allroad and a 2016 Audi Etron with lots of highway miles. On the Etron, I saw at least a 40% decrease in mpg, which was horrible. I had the box slid all the way back as far as possible to eliminate overhang (still could see it if you looked directly under the windshield), but with three people, cargo box, 1.4turbo, and a good headwind, we swapped out for the other Audi. On the Allroad, at most I saw a 5% decrease doing 80mph. The box is suited much better for that longer car.

40% holy moly! I bet the head wind had a large part in the decreased MPG. Thanks for th input!

There's no speed limit advisory in the manual with a roof box fitted? Most manuals I've seen warn not to go faster than 100 kph (60 ish mph). The roof box manual has a similar warning usually.

Not that I’m aware of. I Googled and found nothing for this setup so far. I’m sure a gusts of wind can get you way over 60 MPH anyway. Anyway, I don’t plan to do more than 5 over, USUALLY not to exceed 70 MPH, even if that’s the speed limit.

On my little 01 outback with a somewhat tired engine I noticed my interstate MPG (70-75mph) dropped about 4-5mpg unloaded. I know it's older and not in tip top shape (piston slap) but I even noticed the drag difference in drag when accelerating to interstate speeds.

View attachment 48837

I think you’re giving your engine too little credit. My previous 97 Legacy GT had plenty of pep, all with piston slap that rarely went away after 200k miles!

thanks for the input!

 

[barryh]

A UK magazine actually tested this scenario and were very surprised at the result.

https://www.autoexpress.co.uk/car-news/31861/mpg-mythbusters

Roofbars and box
Getting rid of add-ons when they’re not needed is standard advice if you’re looking to boost your mpg. Why? Because every car is designed to have a low drag, and a roofbox will spoil that. Wrong! Our test with a Thule product showed that even when filled with 72kg of ballast, close to the maximum load limit of 75kg, it only took 0.2mpg off the baseline.
“I’m quite surprised, I thought it would be more,” said Anthony. “I guess it’s because roofbox makers attempt to make them as aerodynamic as possible.” To prove the point, driver Alan said there was no additional wind noise from the box as he was lapping the track.