It is not advisable to use tyres with a lower speed rating than recommended by your vehicle manufacturer. The speed rating (e.g., 'V' in 205/55 R16 91V) indicates the maximum speed the tyre can safely maintain. Using tyres with a lower speed rating can compromise safety, especially at high speeds, and may also be illegal in some regions. Always choose tyres that match or exceed the speed rating recommended for your vehicle to ensure safe driving conditions.