How much range will my EV lose if I use a rooftop cargo box?

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We have an all-electric Volkswagen ID.4 SUV and are planning to drive from Saskatoon to Vancouver Island for a three-week trip. We’ll be four adults and a dog, so we’re getting a roof-mounted cargo box. Will it cause us to lose range? Any idea how much? – Thom, Saskatoon

When it comes to electric vehicle range, roof racks are a bit of a drag, an expert said.

“From reports I have seen, the aerodynamic impact of a rooftop cargo box could [reduce] range up to 10 per cent, so not a big worry given the range of modern EVs,” said Michael Stanyer, communications program lead for Plug In BC, a Vancouver-based not-for-profit EV education program. “Less optimized cargo like rooftop bike carriers or equipment trays (such as cargo baskets) with tall bins will have a bigger effect. Towing a trailer will also have more impact due to poor aerodynamics.”

Air resistance or drag is one of many factors that can affect how much range you get in an EV. Others include cargo weight, road elevation, outdoor temperature, use of heating and air conditioning, and vehicle speed.

Because vehicles vary in size, shape and energy efficiency, it’s tough to give a ballpark figure of how much range you could lose from a rooftop carrier alone, Stanyer said.

For instance, I talked to a driver of a Kia EV6 with a rooftop box at a public charger in Hope, B.C., and he said it reduced his range by about 20 per cent.

“It’s best to get feedback from an online community specific to a given vehicle,” Stanyer said.

Volkswagen Canada said it doesn’t have official data for range loss with a rooftop cargo carrier.

But in a Volkswagen EV discussion forum, one owner said a rooftop cargo box cuts their vehicle’s range by about 13 per cent. That would mean a loss of 53 kilometres on the all-wheel drive ID.4 with a 410-kilometre estimated top range, and nearly 58 kilometres on the rear-wheel-drive version with a 443-kilometre range.

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Think outside the box?

Cargo boxes sap mileage from gas-powered cars, too.

In tests by Consumer Reports, a cargo pod worsened gas mileage in a 2019 Toyota RAV4 by 13 per cent.

With a rooftop cargo box, you’ll also lose range from the weight of the cargo too – because any added weight means you’ll get fewer kilometres out of a full battery.

A study released this year by the American Automobile Association (AAA) found that the range of a Ford F-150 Lightning dropped by 24.5 per cent when it was loaded with 1,400 pounds (635 kilograms) of sandbags.

“While this study may heighten concerns, it’s worth remembering that excess weight reduces fuel economy in gas-powered vehicles too,” an AAA spokeswoman said in a statement.

One way to get better range with an EV rooftop cargo box – and to get better range generally – is to slow down, Stanyer said.

“Speed has a big effect on efficiency and is especially punishing for bulky vehicles,” he said. “There is a dramatic difference between driving on a 90-kilometre-an-hour highway and a 110-kilometre-an-hour highway.”

If you’re planning an EV road trip with a cargo box and you don’t know how much range to expect, use the EV’s GPS – most will calculate how much power you’re using and how much range you’ll have to your destination, he said.

“Modern systems account for your route and energy consumption, providing updated range estimates in real-time,” Stanyer said.

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