The drive train of a truck is modified by the addition of a single changeable pair of mating gears positioned between the output shaft of the transmission and the intermediate gear train that drives the ring/bull gear fixed to the drive-axle differential for a first set of drive wheels. One gear of this changeable pair is releasably connected to the distal end of the output shaft of the transmission, while the mating gear is releasably connected to the input of the intermediate gear train of the ring/bull gear for single drive-axle trucks or to the housing of a high-bias drive-shaft differential having co-axial output shafts for tandem drive-axle trucks. These changeable gear pairs are preferably maintained in stock in predetermined ratios so that the drive ratio of the truck can be quickly, easily, and inexpensively altered by the selection of an appropriate gear pair to accommodate different expected operating conditions.