The American Chemistry Council sponsored program to optimize a specimen design for use in high strain rate testing of long fiber-reinforced thermoplastics (LFRT) was experimentally validated through testing of injection molded long glass-filled polypropylene (LGFPP) and long glass filled Nylon ® (Nylon). It was demonstrated that the dynamic specimen geometry generated valid results for LFRT tensile tests in the quasi-static through 400/s regime. Optimum specimen size depended on the maximum test rates and end use of the data. The program results provide a basis to select specimen parameters to appropriately represent LFRT or similar materials for comparison or material property testing.Tests established the effects of injection technique; strain rate (nominal 0.1/s to 400/s); fiber fill content (20wt%, 30wt%, 40wt%), specimen type and width, panel thickness, distance to the fill gate, flow orientation, and material homogeneity. Not all variables were tested using material from both vendors.For a given LGFPP specimen size and vendor, the strength and modulus increased with fiber content, specimen width, and nominal strain rate. The LGFPP strength and failure strain increased between 0.001/s and 4/s but remained constant between 4/s and 45/s. Macroscopic and microscopic examinations of the fractures surfaces showed a strong mechanical bond between the fibers and the matrix.The Nylon demonstrated no difference in the strength between specimens identified with large or small amounts of fiber clusters. The ultimate strength, yield strength, and modulus of Nylon° increased between 0.001/s and 45/s. The failure strain decreased between 0.001/s and 45/s.