Large exterior body parts such as bumpers, side door panels and rocker panels represent a considerable challenge to raw material suppliers and a tremendous impact on the auto industry. A multitude of partially contradictory requirements have to be met: E-modulus between 500 and 3000 N/mm2 ductile fracture even at low temperatures thermal expansion coefficient to match metal good paintability and Class A surface quality ease of processing on conventional injection molding machines cost efficiency Glass fiber reinforced thermoplastic polyurethanes (R-TPU) meet all these criteria. They are being used by several major European car manufacturers.Glass fibers play a key role in determining the properties of R-TPU. Systematic work on the influence of concentration, fiber diameter and length/diameter-ratio on the mechanical properties has been carried out. The results are compared with existing theories on fiber reinforced materials.In the case of E-modulus a good agreement was found. No theory applies for most of the other mechanical properties, but empirical relationships can be established.A question which is discussed very often is the value for elongation at break. In some cases high values are favorable. It can be shown that the traditional way of realizing high elongations with fiber reinforced materials, namely, using unsized fibers, has severe drawbacks. Their elongations are high only at low test speeds, but at strain rates, which are of technical interest, elongation is reduced when optimum sized fibers are used. On the other hand, good adhesion of the fiber to the matrix is essential for optimizing almost all mechanical properties including tear strength, impact strength and stiffness.