Abstract: A dump truck or other truck having a rigid chassis and a front and a rear axle assemblies secured at fixed points along the chassis, wherein this truck is distinctively provided with a supplementary axle assembly displaceable behind the conventional rear axle assembly lengthwise of the truck chassis, between a position protracting rearward from the latter to increase the payload rating of the truck and a retracted position under the rear of the chassis for unhindered dumping operation. The supplementary axle assembly is also vertically adjustable between an off-the-ground position for the empty return trips, a normal suspension position for the increased payload rating, and a stiffer suspension position for increased loading on the front axle and steering maneuverability.