Easy, make a new layer, use brush tool (about 100-150px in size, white) keep on pressing it to highlight area, but not too much, then use motion blur, and there you go.
I see where you're trying to go with that, looks quite good. The 'frame' needs a little more attention (jagged edges on the uprights) and you missed a little bit of the rendered floor just behind the front wheel.
Thanks
The frame gets a bit more attention next time And i noticed the floor too, but the shadow was a bit too dark for me to see where the lines of the car were (or the car's color was to dark to see the floor:razz, I wanted to have the shadow of the car also "sticking out" of the picture, I tried to cut that out, but it was pretty hard and in the end didn't look good.
Anyway, it was the first time ever I tried this "effect", and wanted to share it here anyway
An idea if you try it again, duplicate the car, then on the duplicated layer, delete the car but leave the shadow and vice versa on the other layer. That way you can then make the shadow a bit transparent and be cast over your frame.