When positioning the top of the car over the body of the car, no offset is needed to make sure they are centered horizontally relative to one another. It looks like this tutorial assumes that draw-rectangle(...) and draw-oval(...) use the first two parameters as the left and top edges of the shape. On my computer at least this is not a valid assumption. I've included a screen shot of what the tutorial's final code produces on my computer.

[maryrosecook]