How Roof Racks Affect Your Car (Gas Mileage & Performance)
Attaching an accessory to the outside of your vehicle changes dimensions, shape, weight, aerodynamics, and with that possibly driving characteristics. Of all the bike rack types, the roof rack is arguably the most obtrusive in this regard.
Car roof racks affect a vehicle’s fuel economy, handling, and performance more than any other car bike rack. This is due to the higher center of gravity and increased areas exposed to wind from the front and sides. The differences between a loaded and an unloaded roof rack are greater than between an empty one and no roof rack at all.
With that being said, that might sound more dramatic than it really is. Some of the effects are not really tangible for you as the driver, while others can be felt in certain driving situations – or on the next stop at the gas station.
Related article: Is it okay to leave roof bars on?
Pro-tip: Racks mounted behind the car have up to 4 times less impact on gas mileage than even aerodynamic roof racks. If you want the most fuel-efficient bike racks, check out the best hitch bike racks on Amazon now.
Let’s face it: There is no way a roof rack will improve your car’s aerodynamics. Adding anything to the outside of your car is going to affect your vehicle’s wind resistance, this is no exemption for a roof rack.
A roof rack affects your vehicle’s aerodynamic drag negatively because the frontal area of attack for headwinds increases. Cargo, like bikes, has the biggest impact as it adds a lot of area exposed to winds from the front and sides. The total magnitude of this effect depends largely on the aerodynamics (size and shape) of the car, the rack itself, and the cargo on it.
To make these factors tangible, let’s look at how the drag force of a car is calculated and what goes into the main equation:
Drag Force = Drag Coefficient * Frontal Area * Velocity² * 1/2
Don’t worry, you don’t need to go out and take a tape measure to your car now! Rather we generally want to understand how mounting a rack on a car roof affects drag – which is the same for all cars.
The drag coefficient is basically a measure of how aerodynamic a vehicle is. A car by itself is already a big, heavy object moving through air and producing wind drag. Some cars are more aerodynamic (drag coefficient of 0,3 – 0,4) than others, while other types of cars like SUVs and vans are as aerodynamic as a garage door (DC 0,5 – 0,6). This is just due to the design and overall shape and nothing we can really influence positively – other than swapping cars entirely.
In any case, an empty roof rack will not add much to the overall drag coefficient, as it has a small footprint compared to the entire car. Same with the frontal area, which is the total area that is exposed to the relative direction of the wind – so from the front to the back while driving.
For loaded roof racks, the story is a much different one. Depending on the cargo, frontal area and the drag coefficient of the entire car change dramatically, especially with bicycles on the roof. This means 2 of the 3 variables relevant for air drag are directly influenced negatively by roof racks.
The larger in surface area of the cargo, the larger the impact on the coefficient of drag. And the more aerodynamic the cargo you strap to your car, the less they affect your fuel economy.
There are aerodynamic roof rack designs available. Some achieve a drag of only 10% in comparison to high-drag models. But what we discussed up until this point shows the little impact the actual roof rack has on both frontal area and drag coefficient. Only the latter is reduced by an aerodynamic design. So the extra cost is probably not worth it overall.
Now, the last variable of the 3 in the drag force equation has a small, but critical number next to it. Wind resistance increases exponentially with velocity. Thus speed has the biggest single impact on drag and fuel economy. Let’s dive in a little deeper.
Fuel economy with a roof rack
Long story short: Roof racks are a drag on fuel economy. (Pun definitely intended)
There is no way around it, roof racks make any car’s aerodynamics worse by increasing the frontal area and increasing the drag coefficient of a vehicle, making the engine work harder against wind resistance to achieve the same speeds.
To illustrate this point, I created the table below showing the drag force value based on changes in velocity – the factor that is squared in the equation. The baseline for 100% drag is at 50 mph or 80 km/h, which is widely regarded as the most fuel-efficient speed for most cars.
Notice that drag does not fall as sharply with reductions in velocity as it is rising with increases in velocity. With an increase of 20 mph, the drag force almost doubles from its value at 50mph. While a reduction of 20 mph only accounts for a 66% decrease in drag.
|Drag Force Percentage||mph||km/h|
This relation between velocity and wind resistance is the reason why velocity has the biggest single impact on fuel economy. The faster you go, the harder the engine has to work in order to fight the increasing resistance. At 100 mph or 161 km/h, the aero drag has quadrupled. This is why this speed is not even achievable by smaller, less powerful cars.
That’s a long way to say, that your traveling pace has arguably the most impact on gas mileage and also is the easiest one to control. So let’s get back to the impact of roof racks.
I’m certain you have seen cars driving around with an empty roof rack. Some of us have even done it ourselves. I never even used to remove my roof rack, not even when I sold my old Honda Civic. They were just a dream couple never to be separated again.
Fuel economy of empty roof racks
Anyway, whenever I see an empty rack, I wonder if the effect on fuel economy warrants removing the rack after use. After some number crunching the answer is rather simple.
With average speeds below 60 mph (100km/h) and in city traffic, there is virtually no loss (1% to 2%) in gas mileage when driving with an empty roof rack. At higher average speeds there is typically a reduction in fuel efficiency of 5% to 15%.
So it is true, an empty roof rack increases the fuel consumption of a vehicle in some scenarios. The small footprint may not be noticeable while driving, but it can be at the gas station.
Fuel economy of loaded roof racks
Driving with loaded roof racks at lower speeds shows a 7,5% to 15% in loss of fuel efficiency. Traveling at highway speeds above 60 mph (100km/h) the average loss in fuel economy is between 20% and 30%. For trucks, it is a little less at roughly 12%.
You probably noticed there is a spread in average values. This is due to the frontal area and drag coefficient of the base vehicle – the two variables we discussed above. Smaller, more aerodynamic cars experience a larger increase in their drag coefficient percentage-wise. While trucks and vans have a higher coefficient to start with and experience a smaller relative impact.
Do roof racks slow you down?
Let’s use what we learned before.
With a given amount of throttle applied, a roof rack does in fact slow a vehicle down by the amount of air drag it generates. The maximum speed the car can achieve is also reduced for the same reason. Additionally, wind gusts have a greater area of attack and impact on speed.
The same can be said for acceleration, though the impact changes with velocity as we discovered before. Accelerating from a standstill at a traffic light does not generate much drag while accelerating in order to overtake on a highway is certainly another story.
Use the “air brakes” on your roof to your advantage too to decrease speed and save some of your brake pads.
Weight increase with a roof rack
An average bike roof rack weighs around 15,5 lb (or 7 kg), which is totally negligible in relation to the weight of a car. The maximum capacity of a usual roof bike rack is around 165 lb (or 75 kg), which is still comparatively light and equals just one additional adult passenger.
So, bike roof racks have a relatively small effect in percentages of the total car weight. One bike adds roughly 1% of the car’s weight. So if you stack the roof rack full of bikes and manage to mount 4, that’s still only 4%. The three additional cyclists onboard make the bigger difference.
Bike roof racks are not the only roof racks out there (roof racks are incredibly versatile). They may have the biggest footprint in size, but other attachments like roof tents, cargo baskets, and roof boxes add the most weight. Your vehicle will have a part in the manual about the maximum weight it can carry. So does your roof rack. Especially with those kinds of heavy weight-bearing roof racks, it’s important to make sure the car isn’t overloaded with it, as this can negatively affect your car’s performance and handling.
The heavier the weight on a roof rack, the greater the negative impacts are on handling, acceleration, braking, and fuel economy. These effects do only increase linearly, not exponentially as they do with aerodynamic drag. The biggest difference can be noticed in deceleration (braking) and sharp turns.
Unless you are driving a drag racer with poor brakes, cars can decelerate much quicker than accelerate. So, while you may notice your car struggling a bit more to get up to speed, this effect is minor compared to the changes you can expect to your overall stopping distance to a complete halt. The braking power required to stop a car going at a given speed varies proportionally with its weight. Since brakes cannot get more powerful than they are, with given braking power the stopping distance increases. Let’s look at an example and take four adults at 75 kg (165 lb) each and 200 kg (441 lb) of their luggage for a trip with a 2.000 kg (4410 lb) car. That 500 kg (1100 lb) of additional weight is a 25% increase and equates directly to a 25% increase in stopping distance.
During braking, the weight shifts forwards and increases load to the front two tires. The same is true for going through turns when the front tires bear most of the lateral forces. This is why there is marginal higher tire wear on the front tires due to heavier weights. So with any significant weight increase, especially for longer trips, make sure the tire pressure is still adequate and increase if not.
Handling affected by heavy roof racks
Any weight transported using a car roof rack leads to a higher center of gravity of the entire vehicle. This is negligible for relatively light-weight cargo like bicycles but is something to consider with smaller vehicles and heavier cargo like tents, roof boxes, or other heavy items attached to the rack. Keep the maximum load capacity of the roof rack and car in check.
So handling is where the additional weight is noticeable, especially in corners and heavy braking. In corners, the car may sway or lean more, while braking distances increase with additional weight. You may also notice that wind gusts from the side and front can have more influence on driving. This is not something necessarily dangerous or damaging, but can certainly be felt while driving.
Heavy loads on a roof rack can also ironically lead to slightly better fuel economy from the car being lower to the ground as it is sitting more into the suspension travel with the additional weight. Two opposing effects are probably not quite canceling each other out. Sitting deeper into suspension travel is in itself not something beneficial and may harm the overall handling, especially on rough roads.
Height problems with a roof rack
This one is obvious, but still not to underestimate. I have seen too many unnecessary (and expensive!) wrecks right at people’s homes.
Bikes on a roof rack can almost double the total height of a car. From the outside, this looks gargantuan but for the driver, the cargo right overhead is pretty much out of sight and out of mind. It is so easy to forget that, especially because it is not everyday that you travel with extra cargo on the roof.
Despite the failed (unintentional) attempts I have seen, there is simply no way to get a fully loaded roof rack into most garages. The same goes for parking decks. These are all inaccessible with a loaded roof rack.
Tunnels on the other hand usually are no problem at all.
Roof racks can be noisy or whistle
Like a solar eclipse, this phenomenon rarely happens, but when the stars align you are in for a special experience. A not-so-pleasant one in this case.
This is called an Aeolian noise and can basically occur when air moves at certain speeds around an object of a certain shape. In our case, it depends on the shape of a roof rack’s parts exposed to wind, the angle at which air hits the rack, and the speed of the air moving by. You can influence all of those three factors either by your driving style or by manipulating the rack itself.
Just think of what you need to do in order to whistle with your mouth: particular lip shape, airspeed, and angle. If one of those is not quite right, the only audible noise is the spit exiting your mouth, but no fine-tuned whistling. Just as learning to whistle yourself requires everything to fall into place, so does your roof rack before it can whistle. Remove just one factor and it will stop making noises.
If you haven’t bought your roof rack, there are roof racks specifically designed to prevent this whistling issue. This is a hot tip especially for cars with a sunroof, where you are exposed directly to any noise coming from your roof rack.
For specific steps to solve this issue, I wrote a separate article on how to stop roof rack noise.