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I have a standard 2 bar roof rack (such as Yakima or Thule) with interchangeable and move-able bike / ski racks. I often carry 2 bicycles. Which will give me the best (least worse) gas mileage on my car:

  • Both bikes as close together as possible, or on opposite edges of the roof?
  • If together, centered, or on one side or the other?
  • Front wheel on (full upright) or front wheel off (fork mount)?
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My gut feel is that fork mount will be a hair better, assuming you stow the front wheel in the vehicle vs on a separate mount. But the difference is unlikely to be substantial. A fairing (eg, tight tarp) over the set of bikes might do better than the bikes alone, but it would take either a wind tunnel or an aerodynamic engineer to tell for sure. –  Daniel R Hicks Feb 2 '12 at 20:16
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Actually, on a clear, flat road, on a relatively windless day, and with a car that has a built-in MPG meter you might be able to have your own wind tunnel. After the car's warmed up run repeated runs the same direction (relative to the wind) with different configurations, and see what the instantaneous MPG is at a steady speed. –  Daniel R Hicks Feb 2 '12 at 22:06
    
Folding bikes don't have this problem. Just saying... :) –  Neil Fein Feb 3 '12 at 4:09
    
I carry my bikes inside the vehicle, mostly because I've heard to many horror stories of people forgetting about the bikes and going under low bridges, garage doors, etc. Anyone I go riding with can ride on the roof. –  lawndartcatcher Feb 3 '12 at 14:55
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6 Answers

Basic aerodynamics, as I remember it from many years ago, would say the only one of those options that would be different from the others would be the front wheel off, but as to whether that would be worse or better I don't know.

With the bike angled down, the seat may have greater drag, and the top tube is no longer horizontal so my guess would be that having the frame horizontal is best (it has been designed to have lowest drag in this position with a rider so this seems reasonable)

As to where on the roof the bikes should be, the most significant drag is from the airflow heading from front to back of the car. There is a minor effect over the edges of the car, but it isn't going to make a noticeable difference, so my guidance:

Don't worry about the drag, and secure the bikes where you can make sure they aren't loose and can get them up and down without straining.

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From my experience frequently carrying bikes, skis, roof boxes, and even 55 gallon drums on my roof rack, I've found that they pretty much all give a hit to the gas mileage, but the configuration doesn't really matter as long as you don't do anything ridiculous.

However, a fairing helps cut down on noise tremendously.

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Do some experimenting with position, but the biggest difference will be by dropping your speed. Just plan on going a bit slower if fuel efficiency is more important than time of arrival.

Whatever the detrimental effects of one placement over another, they'll pale compared to the the improvement gained by cutting 5-10mph off your average speed for the journey.

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Your car's specific geometry will be the determinate factor in the optimal position. Do a couple trials with round trips 10+ miles and in a couple configurations. Short drives won't break the bank and they will help you figure out how to set up your rig for long travels.

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I would not use any type of tarp or cover this would increase the surface area and increase drag and the flapping tarp will damage the paint.I would remove the wheels stow them in the vehicle,again to reduce the frontal area.My experience has been that very aerodynamic vehicles are much more sensitve to drag ,they get high mpg by being moving air over the body very effiencently any disruption makes the smaller engines work that much harder.

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We have a prius. It averages 46mpg on our normal driving trends, but 48mpg on long stretches of 65mph trips with the roof rack, but empty.

Example test: 1 road bike, with a round tubed roof rack, and wind faring. I also had a bug / wind screen bike cover. On a trip of 500 miles we dropped to an average of 35pmg.

Giant drop.

1 bike, 2 bikes, forward, backward, road or mtn, seems to be about the same 10mpg drop every time.

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