If you are buying a car for the first time and do not already have insurance in place to add the vehicle to than normally in Texas you will need to buy an insurance policy and get proof of it just to drive off from the dealership's lot. Texas law (Texas Motor Vehicle Safety Responsibility Act in Transportation Code 601.051) states that a person may not operate a motor vehicle in this state unless financial responsibility is established for that vehicle, so there is no grace period in which to get insurance in TX.
According to the Texas Department of Insurance (TDI) if you are buying another car to add to your current auto insurance policy than that policy might automatically cover it with certain limitations. Insurance companies' policy terms differ so read your policy to find out whether it automatically covers an additional or replacement car.
The TDI states that in general, an additional car usually has the same coverage as the car on your policy with the broadest coverage. For example, if you have two cars - one with Liability coverage only and one with Liability, Collision, and Comprehensive coverages - and you buy a third car, the third car will automatically have Liability, Collision, and Comprehensive coverage.
A replacement car usually has the same coverage as the car it replaced. For example, if you trade in an older car that only had Liability coverage; the new car will automatically have only Liability coverage.
Be sure to tell your insurance company as soon as possible that you have added or replaced a car and which coverages you want. You could lose coverage on an additional or replacement car if you wait longer than the number of days specified in your policy to notify your insurance company.
Since insurance companies' guidelines and policy terms differ so widely regarding adding a new car and if it will automatically be covered for a certain time period or not before buying a car you should contact your insurance company to see if you have coverage for a certain amount of days or if you need to immediately inform them to have coverage. Before driving off in the new car though you will need to make sure it is insured or you will be breaking the law in TX.
If you are a first time buyer for a car and insurance than you should get an idea of what vehicle you want, shop around for insurance and then when you are ready to buy the car contact the insurance company you have decided to use and give them the car's information so that your proof of insurance can be faxed to the dealership.
While you would not have your full policy in hand your insurer should be able to send a binder, a temporary insurance contract that provides proof of coverage until you receive a permanent policy, that you can use as proof of insurance at first.
As you can see there is no grace period in which to get insurance. If you are adding a newly purchased car to your policy (or the new car is taking the place of a car you had on the policy and traded in) than you will need to contact your insurance company before buying the vehicle to make sure your coverage extends to this new vehicle. You will need to find out for how long your current coverage extends and what information they will need to formerly add it to your policy.
If you do not have any insurance policy in place and are buying a car in Texas than you will need to obtain insurance coverage before driving off in the vehicle since it is illegal to operate a vehicle without having financial responsibility in the form of auto insurance or another state authorized manner.
To get started on car insurance shopping for a new vehicle's coverage you can follow this link for quotes from an auto insurance specialist.