This was harder to do than I expected. I wanted to follow the contours of the car more closely, to make it look like the smoke you see in a windtunnel. But that was too slow, and I had to run back and forth pretty quickly, making the streak a little "bumpy." A longer exposure would work well, but my driveway is too bright for that.
Double exposure created in camera:
1) 13 seconds for lighting the car: large softbox to left of camera, White Lightning X-1600, 3 pops at 1/4 power + handheld SB-600 at 1/4 power to light rear of car. Also, the streetlight to camera right is contributing more than I wanted, giving the right side of the car a yellow cast and lighting the background.
2) 30 seconds for the fake "windtunnel smoke" -- created with a Fenix P1D LED flashlight on low power, and me running back and forth as fast as possible
Inspiration is from this image