Derby Monkey Pinewood Derby Wheel Gap and Clearance Gauge The Derby Monkey Pinewood Derby Wheel Gap and Clearance Gauge is a simple yet extremely useful tool. Every derby car builder should have one. This convenient tool helps you to properly set the gap between the wheel and the car body when the axles and wheels are being installed. If the wheel to body gap is not properly set it could create unwanted friction due to the wheel rubbing the body. This gauge will help you eliminate that problem.The opposite end serves as a ground clearance gauge to make sure you have the minimum 3/8" clearance from the track to the bottom of the car.