The information regarding the weight and load of passenger cars can be found in the parameters section of the dataset. To access parameter, please consult the data in ecoQuery (ecoinvent online). Read here if you never accessed before.
There is no automatic way to change the load factor for transport passenger cars in ecoinvent. You can though look into the mathematical relationship used to calculated amount of exchanges (once again access the dataset on ecoQeury, by clinking on an exchange you will be able to see the mathematical formula as well as the comment).
I hope this helps.
If you are having trouble accessing ecoQuery, you can send me an email at email@example.com (screenshots can be of help).
Lucia Valsasina, Junior Project Manager, ecoinvent