FWIW if you're trying to avoid the roof rack, I have put mine on my hitch rack on my truck and it did just fine. I can see the rear wheel sticking out a few inches, the front wheel I tied sideways. A tandem is around about eight feet long before you take off or turn the wheels, minus a foot if you take off or turn the front wheel, minus another foot if you take off the back wheel. A full size truck is about 6.5 feet wide, a light truck about 6.
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Genesis 49:16-17
"Well, well!" said Holmes, impatiently. "A good cyclist does not need a high road. The moor is intersected with paths and the moon is at the full."