Car insurance grace period.

Most auto insurance companies give a grace period of between 10-30 days to report the new car.  If it is a replacement car, the new car will include the same insurance coverage that you had on the old one.  For example  if you only had liability on the old car, that is all you will have on the new car til it is reported and added to your insurance policy.

If it is an additional car, you get the best coverage of any car you insure.  If one is liability only, and another is full coverage, you have full coverage on the new car.  Again, you do need to report it as soon as you can to be sure there is no question of insurance.  Your automobile insurance policy should indicate how long you have to report the new car.

If this is your first car, you need to call an insurance company before you buy the car and work out a deal for a new insurance policy.  they will usually allow a day or two for the new policy, or give you a 24/7 phone number to get the insurance in place immediately.