In my case road tax is paid annually on the anniversary of the date of change of ownership, ie the date that I bought the car. It comes in the form of a bill from the Town Hall and is payable at banks listed on the form; again in my case these are, Caja Granada, Caja Rural or at ATM machines run by La Caixa.
La Caixa ATM machines can debit a La Caixa account directly.
If you pay in cash you do not need an account at the specified banks
On the reverse of the form is an invitation to pay by Direct Debit.
You should carry the receipted paid bill in the car with the other documents in case the GC stop you for any reason.
fb