How Much Bike Racks Affect Gas Mileage (Hitch vs Roof)

There are countless factors that influence a vehicle’s fuel consumption. Some are defined by the car design itself, some by driving style. One factor is accessories like bike racks that are attached to the outside of a car changing its weight, dimensions, shape, and with that aerodynamic properties.

A loaded bike rack affects the aerodynamics and weight of a car negatively, resulting in higher fuel consumption from 5% up to 28%. These negative effects are greater, the smaller, less powerful the car is, and the faster the traveling speed is. An empty bike has only a marginal effect of 1% to 14%.

As you can see, there is quite a large area of variation possible based on the type of car and also the type of bike rack. Sedans are more impacted by racks than SUVs and roof racks have more impact on gas mileage than hitch racks. Most likely you will pick a bike rack for a specific car, not the other way around. So let’s look at what to look for in terms of fuel efficiency.

Best type of bike rack for gas mileage

Let’s get right to the point. What’s better in terms of mpg: hitch or roof racks?

Compared to a roof bike rack, a hitch rack is generally better for fuel economy for all vehicle types. The most impactful factors for fuel consumption are bike rack type, vehicle type, and average speed. Traveling above 60 mph (100 km/h) with an empty bike rack has similarly negative effects on gas mileage as driving below 60 mph with a loaded bike rack.

You might think of all the bike rack types, the roof rack is by far the worst for fuel economy as it creates the highest wind drag right on top of the car. There are many effects of roof racks on a car, and the aerodynamic impact is surely the main one.

Hitch bike racks are mounted behind the vehicle and thus are partially in the slipstream. The larger the vehicle the greater the slipstream effect. However, the bikes on the hitch tray are mounted perpendicular to the wind force, creating a larger surface area and acting like a parachute behind the car.

Trunk bike racks are comparable to hitch racks, although generally slightly worse when loaded because they sit higher, usually above the number plate, so the bikes stick out more above the roof. On the flip side, they are slightly better when empty since they often can fold away.

The biggest impacts on fuel economy can be seen from the actual vehicle type and the average speed. Further down I go in-depth into bike rack aerodynamics. The bike rack type has the least impact of those three factors. Further down I go in-depth into why that is, as i discuss bike rack aerodynamics.

Here are the percentage changes in miles per gallon based on bike rack type and vehicle type at typical city traffic speeds and highway speeds:

below 60 mph:SubcompactSedanSUV
Roof bike rack (loaded)16%15%7%
Hitch bike rack (loaded)14%13%5%
Roof bike rack (empty)3%2%1%
Hitch bike rack (empty)1%1%0%
Bike racks’ reduction in fuel efficiency below 60 mph (97 km/h) – city traffic.
above 60 mph:SubcompactSedanSUV
Roof bike rack (loaded)30%28%19%
Hitch bike rack (loaded)27%25%12%
Roof bike rack (empty)14%11%5%
Hitch bike rack (empty)4%3%2%
Bike racks’ reduction in fuel efficiency above 60 mph (97 km/h) – highways.

Sources: Consumer Reports, Department of Energy, Metro MPG and my personal experience.

Note, that the numbers above indicate the relative (percentage) change in gas mileage based on the vehicle type and average speed. The actual miles per gallon depend entirely on the specific vehicle and may still be lower for a sedan than an SUV.


How to increase gas mileage with a bike rack

From the numbers, you can clearly see that the actual rack by itself has an impact on gas mileage, which is increased with higher speeds. A roof rack is inherently sitting in the wind creating drag, while hitch racks sit in the slipstream. With bikes mounted, the difference gets much smaller.

So, we can look at making roof racks by themselves, and loaded hitch and trunk racks more aerodynamic. That makes also sense since it is completely okay to leave roof bars installed all year long. Same with trunk racks. Hitch racks on the other hand should be removed when not in use anyway.

Here is are six methods to reduce the impact a bike rack has on miles per gallon. I wrote a complete article on this topic (link here). Here are the main take-aways as short as possible:

  1. Slower traveling speed (this is huge, immediate and free)
  2. Aerodynamic roof rack models (check the product description)
  3. Wind fairings for roof racks (such as this Thule one)
  4. Mount bikes on the trays closest to the car (hitch & trunk racks)
  5. Mount trunk rack as low as possible (number plate holder accessory needed)
  6. Dismount roof bike rack (empty hitch and trunk racks have little impact)

Aerodynamic drag of bike racks explained

More air resistance is the main culprit of why bike racks result in higher fuel consumption overall. Further down I dive into the effects of weight on fuel economy, but here’s the summary upfront: Weight increase of a bike rack is negligible compared to the worse aerodynamics.

Let’s face it: Bike racks are a drag on fuel economy. (Pun definitely intended) There is no way a bike rack will improve your car’s aerodynamics. Bike racks make any car’s aerodynamics worse by increasing air drag, making the engine work harder against more wind resistance. No matter what type of rack is used. There is, however, a big difference in the extent of the effect between the bike rack types based on where they are mounted.

To make the factors that contribute to drag force 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 affects overall drag – which is the same for all cars on a basic level.

Drag coefficient

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 smaller cars are more aerodynamic (drag coefficient of 0,3 – 0,4) than others, while bigger 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.

So, this baseline drag coefficient cannot be decreased. But it is important to know the baseline to gauge how big of a relative change in percent the installation of a bike rack results in.

One and the same bike rack results in a relatively large increase of drag coefficient to a smaller, aerodynamic car than to a bigger, less aerodynamic car that already has a high drag coefficient.

Frontal area

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.

loaded roof rack aero drag
Serious wind tunnel testing by Buick.

For loaded roof racks, the story is a much different one. Depending on the cargo, the 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.

Velocity squared

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.

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 Percentagemphkm/h
-96%1016
-84%2032
-66%3048
-36%4064
100%5080
+44%60 97
+96%70113
+156%80129
+224%90145
+300%100161
Aerodynamic Drag Forces with a Baseline at 50 mph (80km/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.

Does a bike rack slow you down?

Any bike rack installed on the outside of a car will slow you down because of increased aerodynamic resistance, less so because of weight increase.

So, let’s use what we learned before.

With a given amount of throttle applied, a bike 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.

Just like using the engine brake of a combustion engine to your advantage, you can use the “air brakes” on your roof too to decrease speed and save some of your brake pads.

Weight load of a bike 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 same can be said about trunk racks. While hitch racks are the heaviest type of bike rack, at 55 lbs (25 kg) on average, the rack alone is still relatively little weight. The maximum total capacity of most hitch or roof racks is around 165 lb (or 75 kg), which is comparatively light and equals just one additional adult passenger. So at a maximum possible weight, you are only looking at an additional weight of 1.3 adults.

So, bike 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 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 may have the biggest footprint in size, but other available roof 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 bike rack. Especially with the kinds of heavy weight-bearing roof racks and hitch 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 bike 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.

Can you drive long distances with a bike rack?

It is possible to travel long distances in a car with a loaded bike rack. Due to higher wind resistance and fuel consumption the range with a single tank filling is lowered compared to traveling without a bike rack. On highways, the maximum range is decreased by 15% to 30% because of higher fuel consumption.

The impact of a bike rack on the range is depending on the type of bike rack, thy type of car, and the average traveling pace. On highways, the range is decreased more than in city traffic.

Julian

I'm Julian and I have been a passionate biker for two decades, both on-road and offroad. I enjoy nothing more than loading up the bike rack on my campervan to explore trails on one of my mountain bikes. I'm also the owner of Suspension Traveler.

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