You want to tow a brake-equipped trailer. Where can you find details or guidelines on the permissible towable mass of your car?

You are correct that the permissible towable mass of your car can be found in Part I of your car's registration certificate. This document includes important technical details about your vehicle, including the maximum trailer weight that your car is allowed to tow safely, both with and without a brake system. The trailer's registration certificate or the insurance certificate do not provide this specific information about your car’s towing capacity. Always check your car’s registration certificate before towing to ensure you do not exceed these limits, which helps maintain safe driving and avoids legal issues.