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Old 08-10-13, 08:13 AM   #1
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Roof Car Racks & MPG

Our prius gas mileage drops 15 mpg with two bikes on a rear rack. I wonder how much of a drop you get with two bikes on a roof rack ?
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Old 08-10-13, 09:15 AM   #2
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I don't think roof racks are as bad. The rear one screws up the air flow across the car. I experienced the same issue with a Prius we had when we moved across the country. Our Mini Cooper Clubman we have drops only 2 mpg even when we've got 2 17' sea kayaks on top.

On the rear rack, the bikes are sitting sideways so they catch a lot of air. On the top, they are facing the same way as the car so less of a profile.
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Old 08-10-13, 09:19 AM   #3
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well, let me do the math

....wind and drag coefficients....

....weight of the bikes....

...gas tank size....

...carry the four...

the answer is:

more, probably.
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Old 08-10-13, 09:40 AM   #4
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More , if driving the car faster, less if driving slower.. or into headwinds..

Increase, in the frontal area, and drag from turbulence, thereby.. effected ..
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Old 08-10-13, 11:54 AM   #5
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Our prius gas mileage drops 15 mpg with two bikes on a rear rack. I wonder how much of a drop you get with two bikes on a roof rack ?
I have only done it with a single bike, but the drop for me was about 3 MPG. Now the roof top cargo carrier made about a 15 mpg difference once (gale force headwinds).
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Old 08-10-13, 02:49 PM   #6
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Our prius gas mileage drops 15 mpg with two bikes on a rear rack. I wonder how much of a drop you get with two bikes on a roof rack ?
My Hyundai Elantra's gas mileage drops ~1 mpg with bikes on the back.
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Old 08-10-13, 03:28 PM   #7
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It could also depend on the car I would guess. On my Tahoe there is very little difference with the bikes on the back, even with 4 bikes and a heavy rack. Could be because the bikes are within the draft of the truck. But on the freeway at 65 I get 17 without the bikes and 16 with. But then 4 bikes and a 120 pound rack add close to 200 pounds and three extra riders in the truck add 450 to 550 pounds more.
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Old 08-10-13, 04:59 PM   #8
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it could also be that the prius is just a big wuss.
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Old 08-10-13, 05:34 PM   #9
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My roof rack has caused a significant reduction in mileage... also with a Prius. Only getting about 40 year round instead of about 44.
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Old 08-10-13, 05:39 PM   #10
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it could also be that the prius is just a big wuss.
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Old 08-10-13, 06:22 PM   #11
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Our prius gas mileage drops 15 mpg with two bikes on a rear rack.
I'm surprised at this, are you sure you didn't also have the car loaded up with stuff? Were you on a trip, and driving mostly on the highway (less good for hybrids)?

We saw very little drop in MPG with bikes on the back of our Echo, but did see a drop when the bikes were on the roof. We usually get 39 MPG, and on a trip to St. Louis and back to CA, with a very loaded car (and bikes on the back), our average was 36.45 MPG.

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Old 08-10-13, 06:28 PM   #12
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My Outback got 33 MPG on a trip up from Long Island (that tank was pretty much pure highway at 75-80 MPH and hit some traffic leaving NYC) with the rack off the roof and the rails stowed, also just me in the vehicle. A week later, I had the bike and rack on the roof, 2 extra people and luggage for 3 inside, got 26 MPG driving to vacation at 70-80 MPH, part of the trip was on the same road but the other direction but I also had an off-highway excursion. Both tanks were equal (the 26 MPG tank) or better than what I could get on the same vacation trip in my previous vehicle (2006 Grand Prix and no bike) so I can't complain.
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Old 08-10-13, 06:46 PM   #13
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I'm surprised at this, are you sure you didn't also have the car loaded up with stuff? Were you on a trip, and driving mostly on the highway (less good for hybrids)?
Yes and yes. 2 people and lots of light camping gear/packs and whatnot. We usually get 48-50 highway, but got 33-35.
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Old 08-11-13, 12:01 AM   #14
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I feel a very safe assumption is that all the figures above are extremely rough estimates with small sample sizes. Its very easy to say a vehicle gained or lost mpg based on one thing but no two roads are the same...

For a more factual than guesstimate answer you would have to keep perfect track of your mileage for thousands of miles with and without a roof rack, bikes, etc. In doing so you would have a better idea of mileage rather than per tank mileage that can easily be skewed.
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Old 08-11-13, 08:13 AM   #15
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In most cases, I think you're right, Clark. But we did, in fact, keep perfect track over thousands of miles, and over the entire course of our trips. I recently stopped recording info on every fill up, but when I did, these are the kinds of data I had:

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Old 08-12-13, 09:31 PM   #16
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Our old Civic (now sold) would get 34-36 at 70 without the rack. The Yakima cartop rack alone dropped it to 30 or so. When I put three bikes on, we struggled to hit 25.
FWIW, I've also carried kayaks, and the effect is much smaller, as you'd expect. They slip through the air like they slip through the water.
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