MPG Impact due to bike rack

That's a good question. I had the same question and I decided to test out a theory about it. Hopefully the detailed information I am providing below will be useful for you and other members.

Background

I have had a Thule roof rack on for about two weeks now. I wanted to get some data before I posted back here.

I have the "Rapid Aero" bars, not the square bars, so my experience is related to those.

I live in Santa Barbara, CA. I have stated in other posts that most of our in-town commutes here are short (i.e. less than 5 miles). As a result, Prius' don't perform as efficiently as they do in a place like Los Angeles, because the engines don't have enough time to heat up to the point that they perform at their peak.

My typical average highway/city blended mileage was around 43 mpg before I got the rack.

Rack Only with Front Faring

When I initially installed the rack, I included a front faring assuming that this would greatly reduce noise and might help with mileage. I noticed an almost immediate drop in blended city/highway mileage down to around 37 mpg. This stayed consistent over the next 5 days of driving. I had expected some drop, but not that much.

Rack Only without Front Faring

I got curious about how the faring might be contributing to drag, and therefore reducing mileage. So I decided to remove it and see what happened. The first thing I noticied, was that the wind noise from the rack was not appreciably louder at freeway speeds without the faring than it had been with the faring. At city driving speeds, there isn't enough wind going by the rack to make any noise, so there was no difference there. I am one of those people who will notice even the slightest noise/rattle/etc. so I definitely would have noticed a big increase in wind noise if there was one.

With the faring removed, I observed that my average mileage began to go up. After 5 days of driving with the rack installed, but without the faring, my blended city/highway milage was about 42 mpg - barely a 1 mpg drop from what I got before I added the rack. Even more surprisingly, when I have my cargo box (a Thule Atlantis 1200) on the rack, I still get better mileage (about 38 mpg) than I was getting with just the rack and faring (about 37 mpg). That is compelling.

Theory that Might Explain My Observations

I'm not an engineer, but my observations line up with what I know about aerodynamics.

A Faring Greatly Increases The Frontal Area Of The Vehicle

An important component of wind-induced drag is something called "frontal area". That is basically the 2-dimensional area of a "slice" taken through the largest portion of the vehicle that is exposed to the airstream. When you add a roof rack, you are increasing the frontal area of your Prius by the 2-dimensional area of the towers and crossbars. This is a relatively small increase. If you add a faring, this adds a large amount of area to the frontal area of the vehicle. This causes a larger increase in drag which decreases your mileage.

A Faring Causes A Major Disruption To The Normal Airflow Around The Prius (Turbulence Induced Drag)

The other thing to consider is turbulence-induced drag. A well designed low drag vehicle like an aircraft, rocket, or a Prius, slips through the air, parting it at the front, and letting it flow smoothly back together at the rear. This produces very little turbulence at the rear of the vehicle and keeps drag low. When you add a roof rack, you are introducing new objects into the slipstream around the Prius, and these cause disruptions to the near-perfect surface airflow around the car (this is called laminar flow). It is these disruptions that cause the wind noise that many people report when they add a roof rack to their car (any car, not just a Prius). Rapid-aero bars have a good aerodynamic profile, so they don't cause as much turbulence, and therefore, they are quiter than square bars. The towers are what they are.

The important thing here though, is that when you add a rack without a faring, it produces only four small disruptions to the laminar flow around the vehicle, at the rack tower attachment points. It seems like the bars are positioned above the normal surface laminar airflow over the roof of the car, and don't particularly disrupt it. The bars create their own drag, butit is above, and independent from the airflow around the cabin. It is also probably not that much since the bars are small and thin.

When you add a faring, everything changes. The faring sits right against the roof of the car, and causes a major disruption to a large portion of the laminar flow over the roof, scooping it up and diverting it away from the roof at about a 35 degree angle to the undisrupted flow coming over the top of the windshield. My guess is that this is causing a very large "back eddy" in the flow over the top of the car, with an associated low-pressure area that induces ALOT of drag. Yes, it could be quieter, because the large disruption generates lower frequency sound than small fast-moving turbulence would, but may actually be inducing more drag. And even with a faring, the rack towers are still disrupting the laminar flow, so you aren't trading off one disruption for another when you add a faring, you are adding a new, large disruption to the already-existing one generated by the rack towers.

Your Proverbial Mileage May Vary

Please note that these observations are potentially accurate and repeatable for people like me, who have the rack attached to their car most of the time, but don't have accessories attached to it. In my case, we are throwing a cargo box on the rack when we take out of town trips and need the extra space. The cargo box itself is extremely aerodynamic, which explains why it is not reducing my mileage that much.

Bikes are another issue entirely. They stick WAY up above the roof, are highly unaerodynamic, and on their own, I would expect that they induce a huge amount of turbulence at highway speeds. In this case, the large amount of turbulence created by the faring's diversion of the slipstream is probably LESS than the turbulence that would be induced by the bikes. Therefore, if you frequently carry bikes on your rack, you are likely better off with a faring, since it will divert airflow up and over the even-less-aerodynamic bikes. The same is probably true for other tall, unaerodynamic rack loads.

However, I would guess that for aerodynamic loads, such as kayaks, surfboards, and Thule rack boxes, you may be better off without a faring.

I hope this information is useful to other members.

 

If you are worried about gas mileage, ride the bike.

If you have to drive, and still want to take the bike, take the bike in the most convenient way, and ignore the hit on gas mileage for those (relatively) few miles. When you are not taking the bike with you, take the racks off. If taking the racks off is too hard, permanently install anchor points. When that is done, installing/removing the rack is easier than putting your seatbelt on.

Here's what I do to all my cars:
http://evnut.com/prius_rack_pads.htm

When the racks are on, the mileage is reduced. When they're off (the bulk of the time) there is no change.

Wanting the best gas mileage is great. But you have to get over the fact that when you use the car beyond a normal sedan, that you WILL suffer a loss of gas mileage. Just deal with it and be happy about the mileage when you are not so loaded. That, or get an EV and don't worry at all about how much gas you're using.

 

Oh, and to answer the OP a bit better... I carry bikes on the roof and on the rear. I probably get a tiny bit worse mileage with them on the roof. But as I said, I don't even think about it. On average - even including carrying all kinds of crap on and behind my car - my mileage is over 50 since the car was new.

 

I notice a 1 or 2 mpg drop, when I carry several bikes on the rear. We always go to our local bike trail and it is only about 8 miles away...most of the time, I do take off the rack if not in use.

 

I just want to weigh in with my experience with two bikes on a hitch rack on my 2011: huge hit in MPG.

Normally I get 40-45 mph going 70-80 mph; with the two bikes back there I got 28-30.

Would love to see some kind of fairing solution.