how many days before this can I buy the new car tax If your vehicle is taxed You can apply from the 5th day of the month in which your current tax disc runs out. Your new tax disc will come into force on the fi rst day of the next month and you should not display it on your vehicle until then.lso how early can I apply for road tax, ie if my car tax is due on the 1st August how many days before this can I buy the new car tax.