>> Another difference(?) might be the use of the term "trailer". In Oz, this is usually either a box-like item attached to the rear of a car (automobile, in N.Am.) via a tow bar (on the car) and a drawbar (on the trailer), or a much larger (up to 40' long) item attached to the rear of a prime mover ("tractor" in N.Am); this latter arrangement is called a "semitrailer" in Oz. <<

All true for U.S., except that here too an automobile is ordinarily called a "car." "Semi-trailer" is ordinarily reduced to "semi" (pronounced sem-eye), but "tractor-trailer" is also common. I rarely hear of the double-trailer arrangements you mention and don't know what the recognized shorthand would be.

As a child, I too thought that auto "fenders" included what are normally called "bumpers," as in "bumper sticker." But that may have been an obsolete usage of my grandparents'. The general definition of "fender" certainly seems to fit, however.