Generally all your unsecured debts goes into your IVA (supposed to deal with all ceditors evenly).
I think a lot depends on wether its HP(ie secured against the car) or a Loan, the size of the payments, the value of the car, and your level of need.
If its a normal loan then really it dont matter, it can just go into the general pot of who you owe money to. (There have been postings though of "loan" companys flagging vehciles with HPI flags wrongly thus making them difficult to sell. Just causes a bit of hassle).
Its if HP and too expensive, your IP would probably not want to put it forward.
If it aint got long to run, then once its finished, those payments may get added to the IVA pot. SO initially yu might not have loads to offer, but later it will increase.
I think also some HP policies may have clauses about what happens if you start an IVA. (like they might take the car back anyways)
SO in the end, it depends on you individual circumstnces, hence the I in IVA
