If I’m behind the wheel, “I’m driving”. But if I am in the back seat or the passenger front seat what am I doing? Am I riding in the car?”
“I rode 20 miles.” (My Dad was the driver.)
“We are riding the through the tunnel.”
Are the above sentences common and correct?
You can, but you have to be sensitive to the context. Anyone reading your two examples in isolation would not immediately think you were in a car. By contrast, when we hear Chuck Berry sing
Ridin’ along in my automobile
My baby beside me at the wheel
it is clear not only that he is in a car, but that he is also driving it.