YouTube Video https://youtu.be/OSNNF3PGJqU
This video gives a step-by-step instructions to fix the speedometer pointer for a 1939-1947 Dodge trucks, 1946-1950 Power Wagons, and 1940-1941 4x4 WWII Military Dodge trucks. A replacement plastic disc (that holds the center piece to the pointer) can be purchased on ebay from Eric's Classic Dodge Truck Parts on eBay
This should be done at your own risk. Improper application of the methods shown may result in the complete destruction of mankind as we know it, or at the very least reduce your speedometer to a broken and worthless hulk.
If you are careful, and your core speedometer is in good shape, you are likely to end up with a nice working speedometer. Any outcome apart from a perfect result is caused by conditions beyond my control.
Please use your own judgment of your skills to do this task and take whatever safety precautions you need to take in order to prevent damaging anything, including yourself.