This study covers the Class 4-8 transmission replacement aftermarket in North America and forecasts market size from 2023 to 2029. The base year is 2022, and the study period is from 2019 to 2029. The study analyzes key medium-duty (MD) and heavy-duty (HD) truck transmission replacement trends in the aftermarket and examines the new, used, rebuild, and remanufactured product segments. Market forecasts are derived from miles driven, usage patterns, replacement rates, new truck sales, and total vehicles in operation (VIO), including a breakdown of the transmission replacement aftermarket by unit shipment, revenue, distribution channel, and price. In addition, the study looks at the competitive environment and the market share of the top participants, including supplier profiles. Traditional manual transmission will decline during the forecast period in favor of automatic and automated manual transmission, which offer enhanced performance and drivability. MD trucks are further along in the shift to automatics compared to the HD truck segment. Competitors in the transmission aftermarket are expected to shift focus from manuals in favor of automatics in the short-medium term.