Composite leaf springs constructed of glass fiber reinforced polymeric materials have been recognized as viable replacements for steel springs. Experience gained in the use of composite leaf springs in passenger vehicles is encouraging and clearly demonstrates the utility of glass fiber reinforced composites for leaf spring applications. This paper discusses a study on the use of glass fiber reinforced composite leaf springs for commercial vehicles, particularly for heavy tank trailer suspension systems. A general discussion on the analysis and design of constant width, variable thickness composite springs is presented. An example is given for the design of a composite spring for one of a tank trailer fleet. This spring is three inches wide with a maximum thickness of about two inches. Several dozen springs were fabricated according to the design and road tested on tank trailers. In general, the road test results demonstrated the applicability of glass fiber reinforced composite springs for trailer suspensions. The composite leaf springs were well received by the trailer operators.