Motor vehicle air guiding device, in particular for a heavy goods vehicle, preferably for a trailer of a heavy goods vehicle, having at least one flexible, flat motor vehicle air guiding element, wherein the air guiding element can be connected to the vehicle with the proviso that it covers at least a part of the surface of the underbody of the vehicle, in particular that it covers a flat area of at least 10 %, expediently at least 15 %, preferably at least 20 %, very preferably at least 30 % and particularly preferably at least 40 % of the surface of the underbody, wherein the flat air guiding element extends across at least 40 %, preferably over at least 45 % of the length of the underbody of the vehicle, in particular of the trailer, wherein the flat air guiding element is stretched or tensioned in the longitudinal direction of the vehicle, in particular of the trailer and wherein the stretching, or the tensioning required for the stretching, of the air guiding element is adjustable, wherein the flat air guiding element at least partially covers at least two vehicle axles or at least three vehicle axles from below and also covers the space between the vehicle axles or between the covered portions of the vehicle axles, wherein at least one tensioning device for adjusting the stretching of the air guiding element, or for adjusting the tension, is present and is arranged at a longitudinal end or in the region of a longitudinal end of the flexible air guiding element, wherein the air guiding element is fixed to at least one tensioning rod extending transversely to the direction of travel or transversely to the longitudinal direction of the underbody and wherein the stretching or the tension can be adjusted by translational displacement of the tensioning rod in the direction of travel or against the direction of travel, and wherein a longitudinal end of the air guiding element is fixed to a counter-tensioning rod extending transversely to the direction of travel or to the longitudinal direction of the underbody, wherein the counter-tensioning rod is preferably designed as a passive tensioning element and is only attached to the underbody of the vehicle in the rear area of the vehicle or the trailer. wherein at least one tensioning device for adjusting the stretching of the air guiding element, or for adjusting the tension, is present and is arranged at a longitudinal end or in the region of a longitudinal end of the flexible air guiding element, wherein the air guiding element is fixed to at least one tensioning rod extending transversely to the direction of travel or transversely to the longitudinal direction of the underbody and wherein the stretching or the tension can be adjusted by translational displacement of the tensioning rod in the direction of travel or against the direction of travel, and wherein a longitudinal end of the air guiding element is fixed to a counter-tensioning rod extending transversely to the direction of travel or to the longitudinal direction of the underbody, wherein the counter-tensioning rod is preferably designed as a passive tensioning element and is only attached to the underbody of the vehicle in the rear area of the vehicle or the trailer.