a preferred embodiment of the expandable truck bed of the present invention employs several integrated elements including : a bed floor 140 , a front wall 120 , left and right sidewalls 110 and a rear tailgate 130 . as shown in fig3 , the front wall 120 is a hollow steel panel with open left and right ends 126 . it is vertically positioned adjacent the truck cabin 10 . secured to the bottom of the front wall 120 is the bed floor 140 . the bed floor 140 is a generally horizontal , hollow steel panel formed by a top sheet 142 that directly supports cargo and a bottom sheet 144 , both of which are joined by a bend at their longitudinal rearward ends such that openings 146 remain along the left and right sides of the floor panel 140 . the top 142 and bottom 144 sheets of the floor panel are secured to the bottoms of counterpart metal sheets of the front wall panel 120 to form continuous space within the perpendicularly joined floor 140 and front wall 120 . finally , the tailgate 130 is comprised of two extension panels 136 retractably disposed within a hollow central panel 134 as illustrated in fig6 . the tailgate 130 is hinged to the rearward end of the bed floor 140 and may be pivoted 90 degrees between an open horizontal position and a closed vertical position . as with conventional tailgates and as will be discussed later , the present tailgate 130 releasably engages the sidewalls 110 . from viewing fig2 , it can be understood that each sidewall 110 is a single unit comprising the following three distinct sections : ( 1 ) a side retaining wall 118 which is vertically disposed and functions as a traditional truck bed sidewall ; ( 2 ) a front extension 116 which is vertically disposed , perpendicularly meets the frontward end of the side retaining wall 118 and inserts into an end opening 126 within the front wall 120 ; and ( 3 ) a floor extension 114 which is horizontally disposed , perpendicularly meets the side retaining wall 118 and inserts into a lateral side opening 146 within the floor 140 . therefore , when the two sidewalls 110 are fully contracted , their front extensions 116 and their floor extensions 114 are fully enveloped by the front wall 120 and bed floor 140 , respectively , such that the opposing side retaining walls 118 , the front wall 120 , the tailgate 130 and the floor 140 define the truck bed enclosure . as the sidewalls 110 are laterally extended , their respective front extension portions 116 slide out from within the front wall 120 , and their floor extension portions 114 emerge from within the bed floor 140 . thus , when the bed enclosure is extended , both the bed floor part 140 and the sidewall floor extensions 114 provide cargo floor support , while the front wall 120 and sidewall front extensions 116 jointly provide frontal retention . a plethora of linear motion systems , both manual and automated , could conceivably be used to facilitate lateral sidewall movement . nevertheless , a couple of suitable alternative slide mechanisms are illustrated and described herein . one is a motor driven mechanism and the other , and preferred , mechanism is hydraulically driven . nevertheless , both such slide mechanisms incorporate two identical , synchronously actuated subassemblies which each operate on one sidewall 110 to achieve inward and outward lateral movement of the sidewalls 110 . for simplicity , the configuration of merely one subassembly of each alternative slide mechanism will be described in succession immediately following . however , it should be again emphasized that still other mechanisms for facilitating lateral movement of the sidewall elements could be employed without departing from the present overall inventive concept . as shown in fig4 , embedded within the bed floor &# 39 ; s top and bottom sheets 142 , 144 are arrangements of rollers 148 that are in contact with the sidewall floor extension 114 . these rollers 148 permit the floor extension 114 to slide laterally within the floor panel 140 while encountering minimal friction . also sandwiched within the floor panel 140 — inward of the sidewall extension 114 — is a nut 240 having a threaded bore . the nut 240 is affixed to the floor panel sheets 142 , 144 to prevent it from sliding or rotating . finally , in threaded engagement with the nut 240 is a screw - threaded shaft 220 that is rotated by an electric drive motor 210 . from the motor 220 , the shaft 220 projects laterally outward through the fixed nut 240 and then through a hole 112 within the sidewall floor extension 114 . coupling the shaft 220 to the floor extension 114 are a pair of circular flanges 226 , 224 that are shortly spaced apart along the shaft 220 about opposite sides of the hole 114 . as the drive motor 210 rotates the shaft 220 clockwise , threaded engagement of the shaft 220 and fixed nut 240 causes the rotating shaft 220 to advance linearly outward through the nut 240 . in turn , the shaft &# 39 ; s inner flange 224 pushes against the abutting metal of the sidewall extension 114 and , aided by the bed floor rollers 148 , the sidewall 110 slides laterally outward to expand the width of the cargo bed . conversely , when the shaft 220 is rotated counterclockwise , the shaft 220 is drawn inward through the nut 240 , and the pressing force of the outer flange 226 pulls the sidewall extension 114 deeper within the bed floor 140 to retract the sidewall 110 . the electric motor 210 is selectively controlled by a user interface module 300 disposed within the cabin area of the vehicle . the user module 300 may be wired ( not shown ) to the motor 210 may operate it wirelessly . regardless , the number of revolutions in which the motor 210 is permitted to rotate the attached shaft 220 in either rotational direction should be electronically limited so as to prevent the shaft 220 from advancing too far inward or outward and encountering undesirable resistance ( caused by motor 210 running into the nut 240 , for example ). however , because of the nut and shaft assembly &# 39 ; s susceptibility to mechanical wear , it is preferable to push and pull the sidewalls 110 by way of hydraulic or pneumatic means . therefore , the preferred embodiment of the present invention employs a hydraulic push / pull device or jack 260 as illustrated in fig5 . this horizontally disposed jack 260 comprises a cylindrical rod 262 in telescopic engagement with a larger diametered cylinder 264 . the rod 262 is selectively extendable from and retractable into the cylinder 264 by conventional hydraulic operation . at one end of the jack 260 , its rod 262 is coupled to the sidewall floor extension 114 via a bolted attachment plate 268 or other suitable securing means . at the jack &# 39 ; s opposite end , its cylinder 264 is similarly secured to a vertical mounting wall 150 which resides within the bed floor panel 140 . fig8 illustrates an example of an overall hydraulic system 20 used to operate the jack 260 . in addition to the jack 260 itself , system components include a fluid reservoir 270 , a pump 280 and a pair of solenoids 274 that are positioned about the vehicle as may be desired . the solenoids 274 are continually connected to the jack 260 via fluid lines 288 . however , they are selectively connected to either the pump 280 or the reservoir 270 directly . a pressure line 284 connects the solenoids 274 to the pump 280 , while a drain line 286 connects the solenoids 274 directly to the reservoir 270 . when a solenoid 274 connects to the pressure line 284 , the pump 280 is able to deliver pressurized fluid through the pressure line 284 and then through a fluid line 288 to the jack 260 . conversely , when a solenoid is connected to the drain line 286 , fluid exits the jack 260 through a fluid line 288 and then through the drain line 286 , depositing into the reservoir 270 . when the rate of fluid flow into the jack 260 is greater than that exiting it , fluid pressure projects the rod 262 out of the cylinder 264 and , in turn , pushes the coupled sidewall 110 outward . conversely , when the net fluid flow is out of the jack 260 , the rod 262 withdraws back into the cylinder 264 and pulls the sidewall 110 inward . solenoid connectivity , which dictates the balance of hydraulic fluid flow between the jack 260 and the reservoir 270 and thus effects extension and retraction of the jack 260 , is selectively controlled through a user module 300 that is wired to the solenoids 274 . residing at the outer ends of the two tailgate extension panels 136 are spring loaded latches 430 which secure to corresponding posts within the side retaining walls 118 to maintain tailgate 130 closure . furthermore , if the tailgate 130 is closed while the sidewalls 110 are being extended , engagement of the latches 430 and the side retaining walls 118 will cause the tailgate extension panels 136 to be pulled out from within the tailgate central panel 134 . an extended tailgate 130 is illustrated in fig7 . of course , the opposite effect occurs to the closed tailgate 130 as the sidewalls 100 are retracted . therefore , without any manual manipulation of the tailgate 130 itself and without the use of any separate attachments , the truck bed &# 39 ; s rear retaining wall automatically adjusts to correspond to the changing distance between the left and right side retaining walls 118 . sets of rollers 138 within the tailgate 130 allow its two extension panels 136 to slide back and forth against its central panel 134 . a handle 160 disposed on the tailgate central panel 134 is operatively linked to the left and right latches 430 . more specifically , as shown in fig6 , parallel pairs of rods 164 , 164 ′ connect the handle 160 to each latch 430 , and a clip 168 binds the two rods 164 , 164 ′ together . a first rod 164 is attached to the handle 160 and has the clip 168 rigidly affixed to it . a second rod 164 ′ is attached to the latch 430 , and it is slidable through the clip 168 . the second rod 164 ′ features successive stop grooves 166 along its length , and within the clip 168 is a retracting pin element ( not shown ) that projects into each groove 166 . as the latch 430 advances inward or outward ( by virtue of sidewall lateral movement ), the second rod 164 ′ progresses through the stationary clip 168 . the force applied to the second rod 164 ′ by a moving sidewall 110 will not allow the clip 168 to rest a stop groove 166 along the rod 164 ′. however , the magnitude of the pull force applied to the first rod 164 as the tailgate handle 160 is being pulled is insufficient to dislodge the clip 168 from a stop groove 166 at which the clip 168 is resting . therefore , when the handle 420 is pulled while the tailgate 130 is closed , an opening force is transferred , via the clip 168 , from the first rod 164 to the second rod 164 ′ and is delivered to the latch 430 . this opening force releases the latch 430 from the retaining wall 118 so that the tailgate 130 is free to pivot open . it is understood that substitutions and equivalents for various elements set forth above may be obvious to those skilled in the art . therefore , the full scope and definition of the present invention is to be set forth by the claims that follow .