Position the 1st car in place and the others will follow.
(The first car is the only one you will see until you paste.)
Right click and set the speed of each frame.

When your happy with the speed of frames,
click on File Edit/Optimization wizard

next

CUSTOMIZE

Click on yes and set the color to the color of the background you will be using.

Continue to save as a gif.
You should end up with something like this.
I put it in a frame box for the demo so
the name won't look like it's coming from nowhere.