Actually I think you will find that the speedo on a 300 Series is electronic. When I fitted the 300 body to my 200 Series chassis and running gear I had to get a speedo transducer for it. You will find that, in the wet, the speedo can sometimes fail due to the connections getting wet. The transducer is just a plug on the back og the transfer box where the speedo cable would normally go and has 2 wires coming from it. The transducer can also get messy inside. They are quite easy to take apart and clean.
HTH
Ivan