Last winter I drove down to Florida in my Prime. On that trip, I got terrible gas mileage. I don't recall the exact number, but it was in the mid-30's mpg. Around 15 or so mpg less than I normally get. At first I thought it was because it was bitterly cold when I started out from Connecticut — single-digit (Fahrenheit) temps. But as I got south to warmer climes, it didn't improve much, so while cold may have been a factor initially, it wasn't the primary factor. The primary factor, I'm convinced, is that I was carrying a bicycle on a trunk rack. This rack carries the bike fairly high, so it was well into the wind. But it's hard for me to believe that the added air resistance per se was the cause. It's a bicycle, so it doesn't have that large of a cross-section to resist the wind. My theory, and what I want to ask for opinions on, is that the presence of the bicycle was interfering with the normal aerodynamics of the vehicle, causing the airflow around the car to become turbulent and thus increasing the air resistance of the car itself. Does that make sense? Has anyone else seen a like effect? This year when I drove to Florida under similar conditions, albeit not quite as cold, I got the normal mileage, but the bicycle was inside the car. That was awkward as there isn't a great deal of room for it. I've since added a roof rack and bike carrier. I haven't had a chance to use it yet, but I'm hoping that it will reduce this problem since air will be able to flow normally under the rack and around the rear of the car, and because the bicycle will be facing into the wind instead of across it. Thoughts?
Being a private pilot myself I can confirm that just about anything can interfere with airflow over an airfoil. Frost is but one example. You wouldn’t think it would matter but It sure can. That said, I think your bicycle theory is spot on. Toyota has obviously done a lot to maximize airflow. Take those little black nubs fore and aft of the side windows. Those are for airflow. If those can affect the flow of air for the better I am certain the bicycle does the exact opposite. Why are bicycles a drag? - CAR magazine
Yup. The bike broke up the laminar flow off the back of the car that Toyota worked so hard to develop. That's a HUGE factor at highway speeds. I read years ago that a motorcycle without a full fairing has about the same CD as an 18 wheeler and the fairing doesn't make it a whole lot better.
Anything that breaks up the airflow is bad for our super-slick cars. I'm afraid your roof rack wont be much better, if at all.
Wind resistance is the single biggest factor in fuel usage at speeds above about 20 MPH. BUT.....the impact increases exponentially with an increase in speed. If you remember......were you travelling faster on that first trip maybe ??
I don't believe so. I pretty much drive the speed limit or very slightly above, and the speed limits on those roads (I used the same route) haven't changed.
Looks like you're right. I did a test run with the bike on the car today. It was only about 80 miles, and conditions weren't exactly the same as last winter, but the mileage definitely took a ~10 mpg hit. (It wasn't a wasted trip since I went to the best brewery in the US.)
Just think of it as a convenience trade-off, and you're still getting better mileage than most cars, especially ones with bikes on the top or back.
I have seen some car model maintenance manuals list roof racks as severe duty in the foot notes. Something to check next time you have it open.
Just a terminology correction, because I see this word misused quite often: the word you're looking for is "attached" flow; that is, the boundary layer follows the surface of the car. At anything except very low speeds the flow field around a car is turbulent, with the exception of the very front of the vehicle.
