Hey cobra they are going to rag on you if you do multiple threads and don't give good nomenclature for what truck you are referring to, see M939 is the series of which there are 3, M939 which is the basic small tires and non turbo 250 cummins, the M939A1 which is big tires 14.00x20 and non turbo 250 cummins, and the M939A2 which is 14.00 tires and turbo 8.3L cummins. Now within each of these series there are different models such as the M923, and a M923A1 and the M923A2, which are all the same basic style cargo trucks but for the differences listed above.