When is it illegal for a vehicle to use a mixture of different-sized tires?

Using a mixture of different-sized tires on the same axle is illegal due to significant safety concerns. When both tires on an axle are of different sizes, it can lead to uneven wear, affect handling, and compromise the vehicle's stability. This situation could cause one tire to have a different grip and rolling circumference compared to the other, increasing the risk of accidents, especially in emergency maneuvers or under adverse driving conditions.

The regulations are in place to ensure that all tires on an axle operate under similar conditions, maintaining balanced performance and safety. Consequently, it is crucial for drivers to adhere to these guidelines to ensure their safety and that of others on the road.