We have a 70lb hound mix that rides URE in the back of the 4runner. It all depends on the dog's temperent and motion sensitivity, like people. She was a nervous wreck at first, as a puppy 6 months old maybe. Now, 5 years old, on easy trails and flat sections of most others she will lay down or sit with her head at the window.
My girlfriend and the dog get out for the rough sections. Without a seat belt or hands to hold on, no dog or sober person will enjoy being thrown around.
Use common sense loading and securing gear so it all stays put, out of the dog's space.
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