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Old 11-28-2007, 09:27 AM   This thread is in the EcoModder Project Library | #1 (permalink)
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Roof rack test: quantifying the aero penalty

Originally written: 03-25-2007, 08:44 PM


Waaaay back in the summer of '06 I did a few runs to look at the effect roof racks have on fuel economy.

Originally, this came up in a thread about a cyclist using his Yaris to carry bikes to races he was attending.

(Note: zooms don't work here - go to if you want to see the bigger versions.)

I finally got around to posting details of the "test" (not exactly rigorously scientific) and some pics on Here's the summary:

From: Mini-experiment: the wrath of roof racks (

The comparison was between:
  • A - no roof racks
  • B - empty roof racks
  • C - a mountain bike on the roof racks
  • Average of all 'A' runs: 55.24 mpg (US)
  • Just adding (empty) roof racks reduced fuel economy to 48.2 mpg (US), or -7.0 mpg / -12.7%.
  • Putting the mountain bike on the rack absolutely destroyed the mileage! Down to 40.1 mpg (US), or -15.1 mpg / -27.3%

Also posted some corroboration about the severity of the effects from other people:

Such as the Prius owner who saw his mileage drop from 47 to 36 mpg (US) with a kayak up top.

And Peakster's mini-experiment posted here at GS about the effect of driving with your hatchback wide open: hatch-propped open vs. hatch-closed

For all the gory details:

Honda mods: Ecomodding my $800 Honda Fit 5-speed beater
Mitsu mods: Oops, I did it again! Bought another cheap, 3-cylinder Mirage. Mods in progress...
Ecodriving test: Manual vs. automatic transmission MPG showdown

has launched a forum for the efficient new Mitsubishi Mirage - fuel efficiency info for Geo Metro owners - electric car conversion on a beer budget
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