The world of Garry's Mod is wonderfully free of the real-life needs that drive us to build vehicles to get around. In Gmod, you can move around as fast as you like without a vehicle, and your feet don't get tired. Ultimately, the only reason to drive a vehicle in Gmod is because it's fun. So builders of Gmod vehicles don't have to worry about safety, fuel economy, cost or other factors, unless they want to.
Building the Vehicle
Spawn a board to use as a chassis. Any reasonably large, flat object from the spawned objects menu will do; a door is a good choice.
Position the chassis horizontally in the air and fix it several feet above the ground with the physics gun.
Attach four wheels to the corners of the chassis using the wheel gun.
Spawn a seat you like from the spawned items menu.
Position the seat on the chassis with the physics gun and weld it in place with the weld constraint. This will be the driver's seat, and the direction it is facing will be the front of the vehicle.
Attach a wheel to each corner on the right hand side of the chassis with the wheel gun and set the controls for both wheels so that the 9 key spins them in the forward (clockwise) direction and the 6 key spins them in reverse.
Attach one wheel to each corner of the left side of the chassis and set their controls so that the 4 key spins them forward and the 7 key spins them in reverse.
Grab the chassis with the physics gun and release it to let the vehicle fall to the ground.