How Roof Racks Kill EV Range: The AVILOO Study
If a customer asks you whether a roof rack will affect their EV's range, the honest answer is: dramatically. But until recently, there was little hard data to quantify exactly how much. A controlled study by AVILOO, the TUV-certified battery diagnostics company behind our testing technology, has now put real numbers on it — and the results are striking.
The Study: VW ID.4 on the Austrian A2
AVILOO's CTO Nikolaus Mayerhofer designed and conducted a controlled test on a VW ID.4 along the Austrian A2 motorway. The goal was simple: measure the real-world energy consumption impact of roof-mounted versus rear-mounted cargo carriers at motorway speeds.
The methodology was rigorous. Same vehicle. Same route. Same conditions. The only variable was the cargo configuration.
Here is what they found.
Roof Rack Results
At 130 km/h (approximately 81 mph), mounting a roof rack on the VW ID.4 increased energy consumption so significantly that the driver would need to reduce speed to 97 km/h (60 mph) — a drop of 33 km/h — just to match the energy consumption of the unloaded vehicle at full motorway speed.
That is not a marginal difference. That is a fundamental change in how the vehicle performs.
Rear Rack Results
The same test with a rear-mounted carrier told a very different story. At 130 km/h, the driver only needed to reduce speed by 7 km/h to match the unloaded consumption figure.
The difference between roof and rear mounting is not subtle. It is a factor of nearly five.
Why the Difference Is So Large
The physics behind this are well established but often underestimated. Air resistance increases with the square of speed. Double your speed and you face four times the aerodynamic drag. This is the single most important factor in EV energy consumption at motorway speeds.
A roof rack drastically increases two things:
- Frontal area — the total surface pushing against the air
- Drag coefficient — how cleanly the air flows over the vehicle
Modern EVs are designed with meticulous aerodynamic profiles. The roofline, the curve of the windscreen, the flush door handles — every element is optimised to reduce drag and maximise range. Mounting a rack on the roof destroys that airflow pattern. The air hits the rack, creates turbulence, and the vehicle must push through significantly more resistance.
A rear-mounted carrier, by contrast, sits in the vehicle's wake — an area of already-disrupted airflow. It adds some drag, but nowhere near the penalty of interrupting the clean airflow over the roof.
What This Means in Real-World Range
To put this in practical terms for UK driving:
A VW ID.4 with a 77 kWh battery and a quoted range of approximately 320 miles (WLTP) will see that range reduced substantially with a roof rack at motorway speed. Based on the consumption increase measured in the AVILOO study, a driver could realistically lose 80 to 100 miles of range on a long motorway journey with a loaded roof rack.
That is the difference between completing a journey on a single charge and needing an unplanned charging stop. For a family heading to the Lake District or Cornwall, this changes the trip entirely.
With a rear carrier, the range loss is far more manageable — perhaps 15 to 25 miles under the same conditions. Still a reduction, but one that rarely forces a change of plan.
Other Cargo Considerations
The AVILOO study focused on empty racks and standard carriers, but the real world involves loaded vehicles. A few additional points worth noting:
Weight matters less than you think. On an EV, additional weight has a smaller proportional impact on energy consumption than aerodynamic drag at motorway speeds. A 20 kg roof rack loaded with 30 kg of luggage creates far more range loss from its aerodynamic profile than from its weight. By contrast, the same 50 kg of luggage inside the vehicle — in the boot or on the back seat — has a negligible impact on range.
Bike carriers are a common use case. Many EV owners are active families who need to transport bicycles. A roof-mounted bike carrier is one of the worst aerodynamic configurations possible — two or three bikes standing vertically on the roof create enormous drag. A towbar-mounted rear bike rack is dramatically more efficient and, based on the study's findings, would preserve significantly more range.
Seasonal considerations apply. UK drivers heading to the Alps for skiing or to France for summer holidays will often load roof boxes for the long motorway stretches. In winter, colder temperatures already reduce range by approximately 10%. Adding a roof-mounted load compounds that reduction. Planning a winter motorway journey with a roof box on a degraded battery could mean range falls to 50% or less of the WLTP figure.
The Dealer Conversation
This is exactly the kind of knowledge that separates a dealer who sells EVs from a dealer who understands EVs.
When a buyer is choosing between vehicles — or accessories — being able to explain the real-world impact of cargo carrying on range builds immediate credibility. Most buyers have a vague sense that "stuff on the roof uses more energy." Very few know that it could cost them a third of their motorway speed to compensate.
Practical Advice to Give Buyers
- Always prefer rear-mounted carriers for EVs when possible
- Remove roof racks when not in use — even an empty roof rack increases drag
- Plan longer journeys with cargo — factor in a 20-30% range reduction with a loaded roof rack at motorway speeds
- Consider a roof box over bare bars — a streamlined box creates less turbulence than an open rack with items strapped to it, though both are worse than rear mounting
- Keep heavy items inside the vehicle whenever possible — weight inside the cabin has far less impact than the same weight creating drag on the roof
This is the kind of advisory content that makes a buyer remember your dealership. It positions your sales team as genuinely knowledgeable about EV ownership, not just reading from a spec sheet.
The Bigger Picture: Range Factors and Battery Health
Range anxiety is still the most common concern among used EV buyers. But range is not a fixed number — it depends on speed, temperature, driving style, cargo, and crucially, the health of the battery itself.
A vehicle with 95% State of Health will deliver its expected range. One with 82% SoH has already lost nearly a fifth of its usable capacity. Add a roof rack and aggressive motorway driving on top of a degraded battery, and the real-world range can be shockingly low.
As we covered in our piece on how driving style affects battery degradation, aggressive driving nearly doubles energy consumption. Combine that with a roof-mounted load, and you can see how quickly the numbers compound.
This is why battery health testing matters. A buyer choosing between two identical vehicles at the same mileage could face completely different range realities. Knowing the actual SoH — through an independent, cell-level test — means both you and your buyer can make informed decisions about which vehicle suits their lifestyle.
The Commercial Angle
Every conversation like this is a trust-building moment. When you can explain real-world range factors with data rather than guesswork, buyers notice.
And when you can back that up with a certified battery health report showing exact State of Health, extractable energy in kWh, and cell-level condition, you have moved from selling a car to providing a service. That is the difference between a one-time transaction and a customer who comes back — and refers others.
The test takes 3 minutes. The certificate is issued immediately. And the credibility it builds lasts far longer than any sales pitch.