I think the OPs question is more about the format of the particular message protocol used by passenger car manufacturers than how to get that data!
And the answer is: There is no "standard" protocol. Assuming you haven't got the bus data definition files (.dbc etc) from the manufacturer, you'll have to log the data, separate the message ids (and any multiplex indexing), then one data byte at a time attempt to work out the likely variable being sent, it's byte length, Endianness,scaling, and offset. For some things this will be easy, as you can control that variable directly (for example engine speed or vehicle speed) for others virtually impossible within a sensible time frame.
You will find however that the manufacturers do share message formats between models and even between manufacturers using the same engine or powertrain (ie, Ford and mazda for example).