Here's a better use of miniature cars and pyrotechnics not endorsed by Mattel (this we can guarantee): rocket-powered Hot Wheels. Even if you choose not to study the step-by-step instructions (basically, just strap a model rocket motor to a Hot Wheels car) at Instructables, you must watch the video we've embedded after the jump. It's what we as kids were trying to accomplish 20 years ago. We think.
Rocket Powered Matchbox! - video powered by Metacafe