This document specifies the method for measuring relative wet grip braking performance index to a reference under loaded conditions for new tyres for use on commercial vehicles on a wet-paved surface. The methods developed in this document are meant to reduce the variability. The use of a reference tyre is necessary to limit the variability of the testing method procedures. This document applies to all commercial vehicle, truck and bus tyres. This document does not apply to: —    tyres fitted with additional devices to improve traction properties (e.g. studded tyres); —    professional off-road tyres.