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MPG, Roof loaded vs unloaded, results
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dobryan
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PostPosted: Thu Jun 23, 2011 5:27 pm    Post subject: MPG, Roof loaded vs unloaded, results Reply with quote

Each Spring break for the last 6 years we (wife, 8 year old son, and I) have driven down to Key West with a friend and camped nearby. This time we had the roof fully loaded.

(Friend Joe on the left, Brendan, and myself)
Image may have been reduced in size. Click image to view fullscreen.


It is a 2600 mile round trip so it was a good test to see how the mpg was affected by the extra drag from all that stuff on the roof. (Inflatable canoe in a super large duffel in the front rack and a cargo bag between Thule bars in the back).

I am driving a 2002 2.5 Subie in an '87 Westy 4 speed. The odometer is accurate to 2% (verified by GPS). I used the same cruise control setting for all the highway miles (70 mph at 3400 rpm) and had about 1600 miles of very consistent driving to get data from (I tossed out the starting and finishing tankfuls since they involved driving at speeds other than 70 mph).

Loaded roof rack mpg at 70 mph = 17.5 during the 1600 miles.

In the following three weeks we again went to Florida twice (to Kennedy Spaceflight Center) to see the Space Shuttle launch. This time the roof rack was empty (except the Thule bars are there, but no front or rear cargo bag). It is 900 miles one way to KSC so I got about 3000 miles of highway data on those two trips. Still using the same cruise control settings and the same road network. (BTW, Route 95 south of Virginia is almost all flat driving so no real hills to affect the consistency of the mpg). (For those adding up the miles we drove for 6500 miles from April 15 to May 17....)

No load on the roof mpg at 70 mph = 19.3 during 3000 miles

For comparison to non-highway mpg, during the next 700 miles of suburban driving (30 to 50 mph) I got 22.0 mpg. (Wind resistance, drag, proportional to the square of the speed is everything).

It was an interesting experiment and a pretty valid one since I was able to calculate mpg over many hundreds (thousands) of miles and many tanks of gas to eliminate much error from variations in how full the tank was at various gas stations. Using the cruise control at the exact same setting eliminated driving style and using the same flat road network made for better results as well.

YMMV. Very Happy
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Dave O
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Last edited by dobryan on Thu Jun 23, 2011 5:36 pm; edited 1 time in total
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edgood1
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PostPosted: Thu Jun 23, 2011 5:34 pm    Post subject: Reply with quote

great info. thanks for posting it!

I dig the louvered windows.
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jackbombay
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PostPosted: Thu Jun 23, 2011 7:30 pm    Post subject: Reply with quote

I too am an MPG nerd! Thanks for posting the data!
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dcdubs
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PostPosted: Thu Jun 23, 2011 7:35 pm    Post subject: Reply with quote

mmmmmm data!!
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Robw_z
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PostPosted: Fri Jun 24, 2011 2:14 am    Post subject: Reply with quote

Thanks for the info! I'm also in the MPG nerd club. Lets round it up and say a loaded roof costs you 2mpg.

-Rob
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speedygeorge
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PostPosted: Fri Jun 24, 2011 7:19 am    Post subject: Re: MPG, Roof loaded vs unloaded, results Reply with quote

I have noticed no difference in MPG with only a large Yakima roof box on van.
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dobryan
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PostPosted: Fri Jun 24, 2011 8:10 am    Post subject: Re: MPG, Roof loaded vs unloaded, results Reply with quote

speedygeorge wrote:
I have noticed no difference in MPG with only a large Yakima roof box on van.


I wonder how much the rear cargo bag contributes to the drag vs the front duffel. My guess is that the front duffel bag has the biggest impact and that the air is so turbulent at the rear that the cargo bag there (or a Yak box) has not as much impact.
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Dave O
'87 Westy w/ 2002 Subaru EJ25 and Peloquin TBD

"To travel hopefully is a better thing than to arrive." Robert Louis Stevenson

Join the Vanagon Rescue Squad on the VanAlert app.
http://vanalert.com/

MD>Canada>AK>WA>OR>CA>AZ>UT>WY>SD
https://www.thesamba.com/vw/forum/viewtopic.php?t=620646

Building a bus for travel in Europe
https://www.thesamba.com/vw/forum/viewtopic.php?t=695371
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