Consumer Reports on car roof top bike rack fuel economy. Not.

[eddief]

http://autos.yahoo.com/news/tests-show-bike-racks-ruin-mileage-130000054.html

[KidWok]

I definitely noticed that when I got my first roof rack. It was on a Honda civic in college and my GF and I were visiting her 'rents in another state. We decided to bring our bikes and foolishly made the mistake of putting those bug bras on them. Not only did the fuel economy go down, it completely killed acceleration. I've paid the extra for a hitch rack to be installed ever since...don't think I lose more than 10-15% MPG with bikes on the back.

Tai

[Gsinill]

As stated in another thread, I experienced over 30% decrease in mpg by putting 2 bikes (no deflectors) on a roof rack going from southern Germany to Tuscany. I have to admit though that this included some "speeding" on the Autobahn and in Italy :)

[MattTuck]

I have to say, I do not see a significant deterioration in mileage with my Subaru outback on the highway. Maybe, going from 29-32 mpg on an average trip to maybe 27-31 mpg with the roof rack. These are fork mounted trays, and I generally only use them with mountain bikes, as I try to keep the Kirk inside the car for other reasons.

Still, I think the Subaru is a bigger car than the Accord in the test, and with the AWD, it doesn't get as good mileage to begin with. I think that as the vehicle gets bigger, and there are more drags on fuel efficiency to begin with, the incremental drag from the bikes makes a smaller contribution to fuel efficiency than on a smaller, more aerodynamic car; in that case, the bike is a more significant addition of drag. I don't have math to back this up, just intuition.

[Bruce K]

But are TT bikes better for fuel economy than road or MTBs? :rolleyes:

It really goes to show how much drag has an effect on efficiency/fuel mileage.

I find it interesting that the fairings for roof racks have very little appreciable effect on mileage. They definitely improve wind whistle/noise but it also demonstrates how important it is to close the air and limit the dead space behind an object moving through a liquid or gas.

BK

[Gsinill]

I have to say, I do not see a significant deterioration in mileage with my Subaru outback on the highway...

Interesting. My car back in the day was a BMW 325 station wagon. Pretty decent car back then. I recall that it was not just the mileage but also the way the car accelerated and handled with 2 MTBs on the roof rack. But again, this included some 100+ mph stretches (we were young AND stupid at that time ;))

[Don49]

With my Ford Escape Hybrid there is no significant decrease in mpg with two road bikes on a Yakima roof rack (Boa fork mount). I used a Scan Gauge to monitor fuel flow at 65mph with and without the bikes and saw no difference. Might be different with a smaller vehicle or at higher speeds.