Use the gradient fill over a larger area from middle to top and then middle to bottom to make the selection spread out the transition of blur so it is more gradual.
I also clip out the car onto it's own layer to set it off from the background so the blur can continue down behind the car without blurring the car itself.
On some I blur the background in pieces to add a bit more depth to the background while still getting the tilt/shift effect.
After you get the tilt/shift down you can clean up the pic and add some action
And get crazy with customizing the car itself
