As an 18-year-old in Illinois, you can drive your dad's car with his GEICO insurance as long as you are a member of his household and are regularly operating the vehicle. However, it's important to note that if you have an accident while driving the car, any personal liability, medical expenses, or other damages beyond what your dad's policy pays will be covered by your own separate auto insurance policy, if you have one. If you don't have your own auto insurance policy, you may be personally liable for any damages that exceed your dad's policy limits.
It's also worth noting that if you plan to use the car frequently or for long periods of time, you may want to consider being added as a driver on your dad's GEICO policy. This will ensure that you have full coverage in case of an accident, and it may also lower your dad's insurance rates since he will be sharing the responsibility of insuring the vehicle with you. Additionally, if you plan to drive the car for any commercial purposes, such as food delivery or ride-sharing, you may need to purchase additional insurance coverage to be fully protected.