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Step 1 Start with the Rectangle Select tool and select the front area of the car (ahead of the wheel arch, from top to bottom of the picture).

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Step 2 Next, switch to the Move Selected Pixels tool and, selecting one of the middle points, contract the size of the selected shape by half (so, if your selection is 100px wide, squash it to 50px).

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Step 3 Switch back to the Rectangle Select tool and now select the middle area of the car, the whole area between the front and rear wheels.

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Step 4 Again, use the Move Selected Pixels tool to squash this selection to half of it's original size.

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Step 5 Then, using Rectangle Select again, highlight the area behind the rear wheels.

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Step 6 Once more, use the Move Selected Pixels tool to contract this to half of the original width.

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Step 7 Now, you need to move all the pieces back together, so that they join up neatly.
To get rid of the left-over space, use the Magic Wand tool and select the transparent area of the canvas. Press CTRL+I to invert the selection and go Image > Crop to Selection (CTRL+SHIFT+X).

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This works for any car as long as the image is taken directly side on.