Here's a couple of quotes from another site that had a similar thread with some explanations of how to do it....
"Start with your normal image, duplicate the background layer.
Switch to the new layer and turn off the background layer (makes it easier to see what you're doing).
Erase everything but the wheels, but make sure you leave a bit of the wheel arches.
Duplicate the wheels layer, erase one of the wheels on each of the wheels layers so you've got the rear on one and the front on the other.
Switch back to the background layer (might want to turn off the wheel layers for the time being)
Select all (ctrl-a in PS) and transform (ctrl-t in PS).
Drag the edges of the picture in so that it's just over 50% it's original width.
Now go back to each of the wheel layers and reposition them to where you think they look best.
When you're happy with the image, merge all the layers (flatten the image) and crop the unwanted stuff off the edges."
and....
"This works much better if there's a neutral background, i.e. trees, fields, industrial buildings, etc. If the rest of the pic contains cars, people or houses then the results are gonna be disappointing as they all get squashed too, and it's very noticable, whereas if a tree looks a little thinner then no-one really notices.
Also, try and get a nice square-on side shot rather than a 3/4 one, it makes life much easier when you're only having to work/think in 2 dimensions, and the results will be much more pleasing."
"Start with your normal image, duplicate the background layer.
Switch to the new layer and turn off the background layer (makes it easier to see what you're doing).
Erase everything but the wheels, but make sure you leave a bit of the wheel arches.
Duplicate the wheels layer, erase one of the wheels on each of the wheels layers so you've got the rear on one and the front on the other.
Switch back to the background layer (might want to turn off the wheel layers for the time being)
Select all (ctrl-a in PS) and transform (ctrl-t in PS).
Drag the edges of the picture in so that it's just over 50% it's original width.
Now go back to each of the wheel layers and reposition them to where you think they look best.
When you're happy with the image, merge all the layers (flatten the image) and crop the unwanted stuff off the edges."
and....
"This works much better if there's a neutral background, i.e. trees, fields, industrial buildings, etc. If the rest of the pic contains cars, people or houses then the results are gonna be disappointing as they all get squashed too, and it's very noticable, whereas if a tree looks a little thinner then no-one really notices.
Also, try and get a nice square-on side shot rather than a 3/4 one, it makes life much easier when you're only having to work/think in 2 dimensions, and the results will be much more pleasing."
Bryn seems to have picked it up quite well though 
