I just installed a Yakima car top rack and transported a 17' sea kayak to my house, a 50 miles trip one way. Without the rack and kayak, I was getting 'about' (dashboard reading) 51 mpg for the previous few days. With the rack only, I got about 50 mpg (60-65 mph freeway). With the kayak added on the return trip, I got 40-43 mpg. I was driving slightly more cautiously on the return trip (55 mph) because a 17' boat is a long load for the little Civic and the wind was catching it.

Edit: I consider the weight of the kayak to be negligible. Its 65 lbs is about the same as half a body stashed in the trunk . It's all about the wind resistance.

 

I put a Thule rack on mine over the weekend, with two bike trays. I haven't actually moved any bikes with it, but I was having a conversation about how much the added weight and drag might cause on mpg with a coworker. I have yet to run an experiment, but will let you all know when I do.

 

That's to be expected I'd say. By far the dominant force to overcome driving is air-resistance, a large canoe and racks will have a very high coefficient of drag, and will add appreciable frontal-area when driving head-on. The situation is much worse however if there is a cross-wind, as the net direction of the wind would be somewhat side on, interacting with the large (17' long) surface area of the side of the boat.

 

I have to say I didn't run a true test, but my last tank I got a below average 46.1 computer indicated tank of gas. Because of the record number of 100+ degree days, my mileage was hovering around 45.7 for much of the tank, but some cooler nights and highway driving at the end of the tank pulled it up to 46.1, even after I'd installed the rack and driven about 100 miles with it.

This weekend, I took a riding buddy and both our bikes on a 50 minute each way drive to a mountain biking trail in McKinney-- and my mileage is down to an abysmal 42-point-something, indicating to about a 3 mpg drop when carrying two bikes on the roof rack and the worse mileage I've ever seen in my HCH. It was hot out, and especially after riding, so the AC was cranked. I thought on the way up we must've been driving into the wind, and I was hoping to recover some of that damage on the way home, but it didn't make a difference.

 

I'm putting a yakima on top of mine too.


Thats great results, I dont get only 42 mpg avg with my HCH1 w/ 5 speed!!