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I was just wondering what would get better gas mileage mounted on an identical car (for instance a 3-series coupe): two bikes on a trunk rack, or two bikes on the roof. Anyone owned both types of rack for the same vehicle, how did your mpg fair?
 

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Elitest thrill junkie
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Roof rack vs a strap-on trunk rack? (which is what trunk-racks are).

The trunk rack will probably fare better in terms of milege, it still puts your bike behind the vehicle where the airflow has seperated aroudn the edges of the car. At the very least it's already turbulent airflow. Both of these mean lower drag as opposed to the bike messing up laminar flow in some other region or simply drag due to the cross-section in the wind. Even if your bike "sticks out" a little bit from both sides it is most likely still in the turbulent airflow, so it would still be less drag than having the bike in the roof rack.

Some things to consider: Roof racks kill milege even when no bike is in there, so consider that. Trunk racks screw up your trunk and eventually rub the paint away/dent the metal. It's an imperfect solution, or better put a "temporary" solution. Using a trunk-rack all the time will eventually screw up your car. I'm sure you could put a towbar on that car, but it will cost you. A roof-rack may be the easiest for you to impliment if you aren't willing to shell out money for a towbar and hitch rack.
 

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Sweep the leg!
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This past summer we took a trip to South Dakota. On the way there I had the Jetta loaded with two bikes and gear inside. At highway speeds averaging 70 mph I was getting ~27 mpg. On the way back we parted company so I had less gear in the car, so I took the wheels off the two bikes and put them in the trunk. With just the frames on the rack there were no longer the wheels protruding past the sides of the car. This greatly reduced drag. On the way back I averaged ~80 mph and was still getting ~28 mpg.

A couple of weeks later I bought a Thule for the roof. During the cyclocross season this fall there were 1, 2 or 3 bikes on the roof at times and at best the milage was ~24 mpg. I don't know if having an extra 200# or 400# of riders or gear in the car effected the milage.

Oh yeah, without bikes the Jetta gets ~28 mpg highway on average if I stay under 75 mph.
 

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on my 06 wrx wagon i have both types
with two DH bikes on top on a trip that was over 500 miles i would avg 18mpg
with two DH bikes on a hitch rack i avg 24mpg on the same trip
with two DH bikes inside the wagon i could avg 30mpg

with them on top i get a huge hit in mpg
 

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Harshing my mellow, man..
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Well, I have a diesel Jetta, and I have a roof rack. It cuts my around-town mpg to about 40, without bikes on top, instead of the regular 42. The couple of road-trips I took with bikes on the roof I averaged about 38 instead of the average 45 or more. I haven't ever had a trunk rack, so I really don't have anything to compare it to. Hope I didn't just make more of a mess with this information.
 

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I have had a Thule roof rack on my '05 BMW 328 and now I have it on my '04 Toyota Matrix SRX. I normally carry either a mt. bike or a road bike on the roof, one up at a time.
I have not notice a drop in mileage with the roof rack on either car.

oddly enough last week I installed the roof rack and got 27 mpg compared to my normal 22 - 24 mpg. I haven't change driving habits. mostly in town with some highway. the only difference was, instead of ARCO I filled up at Valero. could it be the gas? heh!

I'll probably go to hitch on my next rack. It's quite a feat to lift a trail bike up and on to the roof rack on top of a Matrix. Last week end I lifted a 33 lb. trail bike 6 times and twice with a 23 lb Steell road bike, missed the rack on the last lift. oy!

maybe i'll go carbon fiber for next road bike! and after a weigh diet I can get the mt. bike down to 30.
 

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trunk rack definitely better for mpg. not sure about the denting of your trunk and damaging your car. i've had a trunk rack for a while and not major issues. i am now looking at a hitch and a hitch rack though so i have access to my trunk on road trips.

class 1 hitch will run you about $130 (installing yourself) and then a 3 bike rack is anywhere from $119-$350 depending on brand and features. you should be able to do it all for about $250 or $300- still cheaper than a roof rack and much easier to load/unload- similar to a trunk rack.
 

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BMF
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Funrover said:
With a trunk rack you get better MPG's, but I feel A LOT better with the roof rack. It cuts down on the mpg's but I use it for more than just bikes.
Bingo! This is the reason I stick with roof racks. I take about a 10 to 12% hit on mpg in either of our cars which are small - but with a space case and bikes or snowboards/skis we can road-trip and still be in the 23 to 25 mpg range and can park anywhere in the city. A fairing makes a big difference also in terms of noise and to a lesser extent mpg.

If you'll only use the rack for bikes then a trunk rack will save you on fuel.
 

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I have recent data on the mileage affect from a roof rack. Yesterday I drove my STI 400 miles each way to Ray's. Without the rack I average 22mpg cruising at 80. With the rack and two bikes on top in fork-mount trays and the front wheels in the car I averaged 14mpg cruising at around 75 both ways.

So, a huge mileage hit for me. ~$42 in extra gas costs on that single trip. I'm looking into moving to a hitch-mount now.
 
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