[dragonwriter]

> Details might vary between different US states, but to generalize from my own experience, proof of liability insurance is never needed for periodic registration.

In California, it is required within 30 days of initial registration, and within 45 when the previous insurance was cancelled (insurers are required to share this information with DMV.) This is mandatory liability insurance (which you can substitute with a bond, instead); if you are buying a car on credit, the lender will generally require it be covered immediately, and that the coverage include sufficient coverage for the loss of the vehicle, not just legally required liability coverage, because the lender wants to be sure they are paid no matter what happens to the car.

[Projectiboga]

New York has a process where the insurance companies are required to notify the state if the policy lapses. There is exactly a 5 day grace period before an automatic suspension after this.