Legally all a 5 seater car has to carry is 5 x 80 kgs + 30 kgs of luggage: total payload of 430kgs. (from memory)
So say a small 5 seater car weighing 1200 kgs would have to have a GVM of at least 1630 Kgs GVM .
Large cars, RVs etc have much bigger GVMs
You sound a smart guy, muddie: perhaps you could research the relevant ADR and let us know what is the exact minimum payload requirement is for a 5 seater sedan & the average weight of an occupant ...eh? (I'm relying on memory above)
For example the GVM of a Toyota Yaris 1645 kgs and its tare weight is approx 1200 kgs with fuel which gives it a payload of 445 kgs .
IMO its ironic that ADRs say that the average weight of a person is 80Kgs . That said mist lifts specify average person at 75 Kgs!
