All things are not equal. That is the point. There are no family diesel wagons, or even sedans!
I had an Opel diesel in the UK in 1988, equivalent of the Commodore. 55mpg. 20 years down the track and still nothing in Aus. That's why we have a diesel Patrol, and I confirm Briter's mpg figures for it. I dunno why they receive favourable tarriffs, but I think they deserve it. Should be a favourable tarriff for all diesels.