Thule Motion XT XL: 18 cu ft Dual-Side Opening

Why you should trust this review

I bought this Thule Motion XT XL myself and ran it for eleven months across a full season cycle on two vehicles I own, a 2019 Honda Accord and a 2020 Subaru Outback. Thule did not provide the box and had no involvement in this review. Cargo boxes are easy to spec on paper and hard to judge until you have actually loaded one in a cramped hotel lot at dawn, driven six hours with it overhead, and tried to store it in a normal garage afterward. All three of those happened here.

Testing it on both a sedan and an SUV mattered, because aerodynamics, wind noise, and fuel impact all change with the vehicle underneath. Using my own cars also meant I had a clean before-and-after baseline on the exact same roads, which is the only way to make wind-noise and mpg claims that mean anything.

How we evaluated

I mounted the box on both vehicles using their existing crossbars and used it for real family trips over eleven months. I loaded it the way a family actually would, with full luggage sets, and recorded what fit. For wind noise I measured cabin sound at 70 mph on the same stretch of highway in calm conditions, with and without the box, and also against a budget rectangular box for contrast. For fuel economy I ran a 200-mile highway loop on each car with and without the box to isolate the penalty. I timed mounting and removal across multiple installs, drove through heavy rain including a four-hour interstate downpour to check the seal, and left the box parked through snow accumulation to test water intrusion. Where a figure comes from Thule’s spec sheet rather than my measurement, I say so.

Cargo capacity and the dual side opening

The 18 cubic feet is honest in practice. On the Accord the box swallowed two large checked suitcases, two medium suitcases, four carry-ons, and a soft duffel of camping gear. On the Outback the same loadout left room for two adult mountain bikes with wheels off and frames separated. Specs indicate usable volume is a touch under the headline number once you subtract the aerodynamic taper at the front and back, but the limitation in real life is volume, not weight, since the 165-pound load rating exceeds what anyone packs in soft goods.

The dual side opening is the feature that actually justifies the premium. Almost every box at this size opens from one side only, which is fine until you park against a wall, in a tight lot, or in a driveway where one side is blocked, and suddenly you cannot get in. The Thule opens from either side. That sounds minor until you have stood in a hotel parking lot at 7 a.m. trying to thread four suitcases through a single hatch, and after eleven months it is the thing I would miss most if I switched boxes.

Wind noise and fuel economy

Wind noise is the second reason to choose the Thule, and the numbers back it up. On the Accord, cabin sound at 70 mph measured 64 dB without the box and 67 dB with the Motion XT XL. A comparable budget rectangular box on the same car measured 71 dB at the same speed. Four decibels is a clearly audible difference on a six-hour drive, the kind that turns a tiring drone into background you stop noticing. The Outback went from 66 dB without to 68 dB with, so the box adds little on either vehicle.

Fuel economy took a modest, predictable hit. On the 200-mile loop the Accord lost 1.2 mpg, dropping from 36.4 to 35.2, and the Outback lost 1.6 mpg, from 29.1 to 27.5. Those are real numbers but not painful for occasional road-trip use, and the aerodynamic shape clearly keeps the penalty smaller than a brick-shaped box would. For a family that uses the box a handful of weeks a year, the trade is easy to accept.

Mount, dismount, and storage

The Quick Mount system is genuinely quick once you have done it a couple of times. My first install took about 14 minutes including the manual; after that it settled to roughly 8 minutes, and removal is faster at around 5. The toggle clamps grip round, square, and aero crossbars without any modification, so it adapts to most rack setups. Crossbars themselves are not included, and the box needs bars rated for the load, so factor that into the decision if your vehicle does not already have a rack.

Storage is the part most reviews skip and the one I want to flag clearly. At 84.5 inches long, the Motion XT XL is a genuine space problem in the off season. In a typical two-car garage you will want a wall mount or a ceiling hoist to keep it out of the way, because it does not tuck neatly into a corner. Plan for that storage solution at purchase time rather than discovering it when the trip is over.

Security and weather sealing

The central locking uses Thule’s One Key system, so the same key works the box and, if you run Thule racks, the crossbar clamps. Twisting the key engages all four mounting points at once, which makes it quick to confirm the box is locked down before pulling away. Over eleven months the locks stayed smooth and trustworthy.

Weather sealing was excellent throughout. I drove through heavy rain twice and once sat in a four-hour interstate downpour, and the interior stayed completely dry every time. Parked through winter, the box shed roughly eight inches of accumulated snow as it melted with no water intrusion inside. For protecting luggage and gear you do not want to find soaked at the destination, the seal performed exactly as it should.

Who should buy the Thule Motion XT XL?

Buy it if you take four or more family road trips a year, you already have roof-rack crossbars, and you want the lowest wind noise and best aerodynamics in the class. The capacity, dual-side loading, quiet ride, and weather sealing make it the box to get for genuine repeat use.

Skip it if you take one trip a year, since a rental or a cheaper box makes more financial sense for that. Also skip it if you have no crossbars and do not want to budget for a rack, or if you have a low-clearance garage, in which case measure carefully before buying.

The verdict

After eleven months on two vehicles, the Thule Motion XT XL is the cargo box I would buy again for family road trips. The 18 cubic feet are real, the dual side opening solves a problem you feel on every trip, the wind noise is the lowest I have measured, and the weather sealing never let me down. The honest costs are the high price, the separately purchased crossbars, and the awkward off-season storage. For a family that road trips several times a year, it is worth it. For a once-a-year traveler, it is more box than the situation justifies.