With the introduction of the AT545R, Allison Transmission completes its family of transmission integral retarders. These retarders, as an auxiliary braking system to vehicle service brakes, will now be available with all commercial on-highway and off-highway transmission models. The AT545R has a retarder located on the input side of the transmission that is designed for auxiliary braking for downhill speed control. This 160-hp retarder has a two-stage capacity that, in conjunction with selection of the appropriate gear ranges, can be used to tailor desired retardation performance to particular duty cycles. The design, development and performance of the AT545R are targeted for school busses and motorhomes, though many other users will find distinct vehicle control advantages with this low cost, compact retarder. The development from concept through production implementation will be discussed in this paper.