I read an interview with one of the directors, he said it's all real no cgi at all, the tyres were weighted with nuts and bolts from the car. Although the action is all real they didn't have a studio big enough to do it all in one take so they used cg to match two takes, the seam is where the exhaust muffler is rolling along the ground.
"They're very much like scruffy pigs to look at, and they've got big, knobbly warts and lumps all over their long, hairy faces. They are very, very ugly indeed..."
