cold impact - resistant tubing especially useful as a fuel line for motor vehicles which is stable in length , has a brief thermal overload capacity , and consists of at least three layers of at least two different mutually compatible polyamides . the tubing also preferably has an internal and an external layer of impact resistance - modified polyamide , with or without a plasticizer , and a barrier between the layers of impact resistance modifier - free homopolyamide , copolyamide or blends thereof .