I think "fleet" and I think "Ford Taurus." Apparently Clarkson thinks "repmobile" and he thinks of a 3-series diesel. My, how things are different.
Well good MPG hence the oil burner, and a roomy car with a certain air of quality (while not being flash) are what you need for cruising the motorways selling stuff to people. A 3-series might be seen as a bit too... you know... sales rep's car these days for any sales rep to bother with one.