6 Ways To Improve Gas Mileage With A Bike Rack
Attaching any bike rack to the outside of your vehicle affects the fuel economy negatively by as much as 28%, depending on the car. By understanding what factors result in this, you can manipulate one or more to your advantage.
There are two main factors contributing to the reduced gas mileage due to a bike rack: the bike rack type, and average speed of travel. Both are generating aerodynamic drag force, which is the main reason bike racks are bad for fuel economy. Changes in speed or mechanical properties can improve fuel efficiency with bike racks.
Aerodynamics is a huge part here. For a detailed look, I got an entire article on why and how bike racks affect gas mileage.
1. Lower your traveling speed
Slower speeds are by far the most impactful way to improve gas mileage while also being the easiest, most immediate, and cheapest method.
This is due to velocity contributing to air drag force exponentially, meaning with double the speed, wind resistance quadruples.
More on the aerodynamic drag of bike racks here.
You can observe the immediate impact live on your car’s dashboard if the current gas consumption or gas mileage can be displayed. If I have a bike on my trunk rack I tend to go about 5 mph (8 km/h) slower on highways. Depending on the strength of the headwind, I vary a little up or down. Strong headwinds are definitely more noticeable with a bike rack in use. For me, highways are also where I travel for the majority of the distance for a bike trip.
So whatever small changes I make on highways translates into big results at the next stop at a gas station.
2. Aerodynamic roof rack models
Reducing roof rack air drag is similar to stopping them from making noise since the physics for both are similar. Roof rack manufacturers develop solutions to eliminate whistling and reduce wind resistance without any extra steps from your side. If you are not yet the proud owner of a roof rack or cargo basket, consider also factoring the aero properties into your buying decision. There are roof racks – specifically the crossbars – specifically designed to prevent this whistling issue.
Look for product descriptions like “aerodynamic and quiet”, “includes wind fairing”, “wind diffusion”, or “wind tunnel tested”.
3. Wind fairings for roof racks
A wind fairing is basically a spoiler in front of the roof rack to change its frontal profile and break wind before it hits the actual rack frame. This achieves two things: a more aerodynamic shape exposed to wind and redirects airflow over the crossbars instead of beneath, which creates disturbances.
If a wind fairing doesn’t come with your roof rack, there are many aftermarket options available from first party manufacturers like Thule, Yakima, and Rhino Racks. The easiest to find and fit are of course offered from the same brand as your rack. Since empty roof racks have little impact on fuel economy, the potential positive impact of a wind fairing is rather limited. Even more so on a loaded one.
4. Use the first tray to load a bike (hitch & trunk racks)
Most bike racks have a capacity of 2 or more bikes. While on roof racks, each bike has the same amount of wind exposure, on hitch and trunk bike racks it makes a difference in which of the trays you actually load your bike.
With bicycles being mounted perpendicular to the direction of travel, they act similar to parachutes hanging from the back of the car. But they are also partly in the slipstream of the car. So, let’s minimize the parachute effect and maximize the slipstream by mounting the bike as close as possible to the car’s body.
This way the airflow is directed more over the bike than over the trunk door and down between bike and hatch.
5. Mount trunk rack as low as possible
As you can see in the picture above, the handlebar, saddle, and even some of the frame protrude above the car roof. That’s a lot of additional exposure generating wind resistance. That large increase of surface area from bikes perpendicular to the direction of travel is exactly what makes trunk racks similarly bad for gas mileage as roof racks.
Hitch racks are mounted lower near the bumper and can take more advantage of the slipstream coming from the car. With a given trunk rack you cannot exactly choose freely how they are mounted.
With that being said, the range of available trunk bike racks for a particular car is not exactly numerous since they often are designed to fit only one model of car. There are, however, more universally fitting trunk racks.
If you have the choice, a trunk rack sitting so low that bikes cover the license plate is certainly the most fuel economic one. Obeying street laws and making the number plate visible can still be done using a simple number plate holder accessory (like this illuminated one from Thule).
6. Dismount roof bike rack
Roof racks are the only kind of bike rack that by itself affects a car’s aerodynamics negatively. As a result, this translates into worse gas mileage just by having one installed.
The same is not true for trunk racks and hitch racks. They hardly affect fuel economy by themselves. For a detailed gas mileage comparison of bike rack types and car types, I got an entire article about that.
With that being said, I should mention that hitch racks should be dismounted anyways when not in use. Partly because they are cumbersome for parking and trunk access, but also because it may very well be unlawful to do so.