Everyone wants a car these days, personally I believe if your wanting to pop in a vehicle then make sure its not the centre of attention of the map, more an added bonus unless your making a car map thing, hell, do what you want anyway

1. Make a vehicle facing west in top view

2. Take the origin brush, make a block and place it in the middle of your car, middle in both width and height seems best.

3. The origin brush is the pivot of the car you see.

4. Now select every bit of you car, and the origin block,

5. Now everything is selected, tie it to the func_vehicle entity.

6. So now you have a vehicle with the origin brush located in its middle, dont worry it becomes invisible.

7. Now the entity properties, set them to something like this:

(obviously change speed etc to what you want)

Vehicle name - car1
Render FX - Normal
Render Mode - Normal
FX Amount - 255
FX Color - 0 0 0
Pitch yaw Roll 0 0 0
First stop target - Stop 1 (for the paths, we discuss this later)
sound - car1
length of vehicle - it automatically does this for you so leave it
width of vehicle - same as above
height above track - ive found you need to play around with this is get in level with the floor. Try some different values and see where it gets you, try say 10 to start.
Initial Speed - 0
Speed - 600
Acceleration 1-10 - 8
Damage on crush - 50
Volume - 8
Bank angle on turns - 0
Minimum light level - just leave it blank

8. Now make some paths, entity is path_track, place them or try to place them inline with the origin brush, im not sure if you need 4 of them but it doesn't seem to make a difference. in the path track properties you only need to put in name and next stop target, so it will be as follows:

path_track 1 (stop 1 in pic)

name - stop 1
next stop target - stop 2

path_track 2

name - stop 2
next stop target - stop 3

and so on.. (link the stop 4 next target to stop 1).

9. So now we have a func_vehicle with its path tracks. All we need now are some controls. You should of entered a name for the car earlier when we filled in the func_vehicle, and that's important here. Now get the aaa texture and great a block where you want the controls, (ive found sometimes it goes weird and you have to move it about a bit which is annoying, but that's the way it is, Its suppose to be at least 64 units high) now tie your aaa block to the func_vehiclecontrols. Then in vehicle name put the name as car1, so it knows which car its controlling. (the vehicle control aaa block will become invisible in game)

10. And that's that, ive found you may have to play with the vehicle height to get it working nice, and the vehicle controls sometimes need moving.

11. Go and drive to the shops in your new made car.

(please note, you cannot add other entities such as lights to a vehicle)

(I do not take responsibility for the crashing of vehicles and hurting of peeps, always wear a seatbelt and watch out for crossing animals)