Snowchains attach to the drive wheels of a vehicle or special systems deploy chains which swing under the tires automatically. Although named after steel chain, snow chains may be made of other materials and in a variety of patterns and strengths. Chains are usually sold in pairs and must be purchased to match a particular tire size (tire diameter and tread width). Driving with Snowchain reduces fuel efficiency, and can reduce the allowable speed of the automobile to approximately 50 km/h (30 mph), but increase traction and braking. Some regions require chains to be used under some weather conditions but some areas prohibit the use of Snowchains which can deteriorate road surfaces.